Picking up a used car instead of buying new could mean getting a much better deal with your budget. Or it could mean getting a much worse deal when you accidentally lumber yourself with a car-shaped bundle of regrets churning out post-warranty maintenance costs, frustration and heartache.

That's why you want to make sure you know what you're doing before you start doing stuff.

Getting a grip on the used market is even more important if you're looking for a BMW. Like other German luxury performance vehicles, the cost of BMW ownership without a full manufacturer warranty can get pretty expensive. The upside is the other affliction of German luxury performance brands: depreciation. Values drop like crazy as soon as they're driven out of the showroom, which is a nightmare for the first owner, but as a second or third owner you have the chance to pick up a real bargain if you tread carefully.

If you've already bought a few used cars, you'll know exactly what to look for. But if you're used to getting your stuff shiny and new and fresh from the factory, moving into the used market — whether you're looking to save money or blow the kid's (or even your) college fund on a high-end out-of-production model — can be pretty daunting.

So here's how it's done.

1. Figure out how much money you have

First, you've got to set a budget. That's the easy part. If you're having any trouble, just ask your SO how much you're allowed to spend.

Sticking to your budget is where things get a bit tricky. Anybody can say they've got $16,000 to spend when they're sitting at home making empty promises to themselves, but when you're taking the car you've haggled down to $19,000 for a test drive and everything's just right, that extra $3,000 suddenly feels like it's no big deal.

To keep yourself from stumbling into for-that-kind-of-money-you-could-have-just-bought-new territory, force yourself to suppress your wandering eye for a moment. The 'max price' filter on basically any decent car listing website is there for a reason. Use it.

Just remember that running costs on a used car are likely to be higher than if you were buying new. You'll have worn-out parts just waiting to give up on you, and when they do, you probably won't be able to get any work done on the factory warranty. A good policy from a reliable third-party warranty provider can give you the same coverage you'd get with a new car, but without one, you'll be paying any repair bills yourself. In that case, budget a little low to make sure upfront costs don't eat into your maintenance budget further down the line.

2. Decide what you're actually looking for

You've probably already got at least some idea whether you'd be looking for a Z3 for the weekend, an M3 for the track or an X3 for the brood.

When you're buying new, being as vague as that will get you a long way. When buying used? Not really.

Once you've figured out what kind of car you're in the market for, you'll have to start getting a lot more specific pretty quickly. What kind of mileage could you live with? Do you care if the service history is a little patchy? Is a scuff mark or two on that Coral Red interior a deal-breaker?

And what are you going to do when something inevitably goes wrong? If you've got an '05 325i, you won't have any problem picking up spare parts or finding a few tips online. With an '05 645Ci it might be a bit hit-and-miss, but you'll probably be fine enough. With an '05 116i... good luck, MacGyver.

Take a look online at what parts you can still find for the model you're thinking about, and check what owners have to say about reliability. You'll always be able to find somebody saying the water pump is guaranteed to go at 60,000 miles, and some solid googling should let you know just how accurate that is.

Just keep in mind that "I had one in college and drove it 250k miles and all I did was change the oil a couple times and it was all good, no problems" is an anecdote, not a professional reliability report.

3. Find a (non-shady) place to shop

Good news: it's the 21st century. That means you don't have to traipse around a load of shady used car lots hunting for a gem. Just jump on eBay and all the shady guys with used cars to sell will come to you. How convenient.

Just in case you aren't comfortable buying a car in cash from a guy who wants the deal to go down in the parking lot of the Kmart just off that intersection on the edge of town, there are plenty of options that are a bit more on the official side.

BMW's Certified Pre-Owned program lets you pick up what's probably going to be a legit car, with a decent warranty to boot. There's no guarantee that it's actually going to be in that good shape (NB not every dealer is a saint among us mere mortals), but at least if they do sell you a dud, they'll have to deal with it for a while. So that's a kind of victory. Then again, you'll be paying a premium compared to independent retailers.

For a middle road between random internet guys and the all-inclusive package, take a look at an independent used car dealer. Just make sure it's one of the reputable ones with decent reviews, otherwise you're basically back to buying from random internet guys.

Even better (maybe), check if any local car rental companies have old stock available. Used car dealers suck up cars from everybody, and not everybody actually takes care of their stuff. People drive rental cars like they stole them, too, but rental companies aren't in the business of constantly shelling out on new cars to replace the ones that have been driven into the ground, so most have pretty decent maintenance regimes for their fleets. That might be just enough to offset the rough daily treatment if you can get a good price, but give everything a once-over before you even consider committing.

4. Inspect the car for any signs that it may in fact be junk

When you've found the right car in the right condition with the right mileage at a good price, don't just go all Monopoly about it throwing cash around. Stop. Wait. Breathe.

It's easy to convince yourself that a five-minute test drive is all you need to figure out a car's running condition, but the reality is pretty much just no. Yes, take it for a test drive to see how you like it, obviously. Then get somebody who knows what they're doing to do a proper pre-purchase inspection before you part with any cash. It's only going to add $100 or so to the cost of the car, so it'll pay for itself soon enough. Promise.

And don't think that just because you're buying from a proper dealership instead of from some guy in the classifieds, you can afford to be less stringent with the inspection. Even CPO vehicles can roll out of the showroom with issues that might be a price worth paying for some people but more effort than it's worth for you. It's always up to the purchaser to figure out what they're getting into and decide whether they can be bothered getting into whatever they're getting into.

One advantage of going to a dealership (whether it's CPO BMW or independent) is that you're far more likely to be able to wangle a 24/48-hour test drive. There are a lot of issues that won't show up until you start putting some strain on the car, so take advantage of an extended test drive whenever you can. Spend a few hours buzzing around the city, then put down some highway miles. It'll give you a much better idea of how your new car is going to cope when you put it to use.

5. Make sure you actually buy the car

In all the excitement of finally finding your new dream car, you might forget that there's plenty of boring stuff that needs doing, too. Stuff like actually taking ownership of it.

If you're buying from a private person, get the title signed over to you before you part with any cash, otherwise... well, it should be obvious what kinds of problems that could cause.

If you've opted for a dealership, you're going to have plenty of paperwork to wade through, too. Everybody knows that nobody reads stuff before they sign it, but make yourself the exception and definitely do that. Keep an eye out for any extra charges the dealer's trying to sneak in under the radar. Check and double-check if there's a return policy if you change your mind. Find out whether there's anything you can do if the cylinder head cracks on the drive home (apart from leaving a 1-star review on Yelp).

It's just what grown ups have to do before they get to play with their new toys.