BMW Buying Tips
So You Want a BMW?
And who can blame you? German build quality, great rear wheel drive handling, style, what more could anybody want from a car? But not all BMWs are born equal. There are some models you want to avoid. A BMW that suits the man who does 40,000 miles on the motorway each year is not what someone like me who drives mainly on twisty country roads.
Please note that much of the stuff below is specific to the UK market.

E39 530i touring has loads of space but with pace too.
How Big?
I'm sure many people would prefer a coupe but if you have a few kids (I find cats are less bother and cheaper incidentally) you'll want four or even five doors. Unfortunately BMWs seem to get bigger every passing year so space probably won't be a problem for you.
The recent three series are a good place to start. Plenty room in the front back rear seat leg room but headroom can be restrictive for larger drivers. Even in the front anyone much over 6ft will likely be uncomfortable. Be aware that the Coupe has a lower roofline than the saloon. The old E28 five series is roughly similar to the E36 but with more rear leg room, that's how much threes have grown.
The older E30 3 series are a lot smaller and lighter than the E36 or later model threes. I prefer this as the car is more fun to drive as a result. The difference in width is very noticeable. Headroom in an E30 is similar to an E36 Coupe but much less than an E36 saloon. My own preference is to run an e30 318iS as a summer car (with cheap classic insurance) and run an e36 328i touring in the winter on snow tyres. I paid about 1200 quid for each car in 2006. I prefer to find them in the south of England as they don't see winter salt and rust a lot less as a result.
The E34, E39 and new E60 five series are bigger of course. I find the biggest difference is the extra elbow room. Lots more space in the back too but still a bit less than the equivalent Merc. A very nice driving experience on almost any road except the narrow country ones I favour. I like the way the extra weight soaks up all those bumps yet still feels stiff and sporty, a masterpiece of design. Of the three the E34 is the best built, possibly the best made BMW ever! The E39 is a bit cheaper but still great, major issue here is the way it chews up suspension components every 60k miles.
If you want more room then the seven series is for you. Why would you need more space than an E39? Let me give you an example. A good friend of mine is, er, plump. On a visit to the local stealer (don't get the joke? you've never had a bill for a BMW dealer oil service) he first sat in an E36 325i saloon, a bit cramped he decreed. Next he tried a five series, better but still a little tight. Then he tried a 730i. Perfect, all the room he needed - shame he couldn't afford it really.
The biggest disappointment space wise must be the 8 series. The old E24 six series used the E28 chassis and actually seemed to have more internal space! A really roomy grand tourer. The eight is a big let down in this regard. It looks much longer than a six yet the back seats aren't much bigger than a 911! Can anybody guess why it never sold very well?
Don't be fooled by the X3 and X5. They have tiny boots compared to a five series, most 4x4s seem to. A five series touring beats an X5 every day in my book. More fun to drive and much cheaper to own.
As for the current six series it's a really competent grand tourer but it's looks do it no favours. I think a used Aston Martin would be a better way to spend your money.

E63 645i V8, an awful lot of car for the current price.
How Fast?
How much speed can you afford? Bigger engined BMWs cost more to insure and use more fuel than their less cubically endowed bretheren. If you move up to a car with more cylinders servicing is slightly more expensive (more sparks plugs to replace, takes more oil, etc.).
On the other hand big V12 powered 750s are fairly cheap to buy, if not to maintain. Few people can justify the fuel consumption and high insurance so for a driver with a clean licence and 5 years no claims bonus they might make sense if you want all that smoothly delivered power and don't do a high annual mileage.
There is little point in buying a big engine if you never get the chance to use it's power. I'd like a more powerful car but it wouldn't help around town and I enjoy getting most out of my 140bhp e30 318iS by careful driving on the "twisties". Of course if you go to track days or do a lot of high speed commuting the effortless power of the new M50 2.8 will definitely appeal to you. The current generation of BMW six cylinder engines is very efficient and make the larger cars a joy to drive. If I'm doing a long main road trip the 328i tourer with it's cruise control gets used. Many find older BMWs more fun as you can find "the limit" as less insane and illegal speeds.
The turbo diesels are superb cars and respond really well to chipping. If you're buying a BMW for any purpose except as the summer fun car then it's silly not to get the diesel.
For the summer / fun car you don't always need the biggest engine. The Z4 2.2 is a great little car to drive, it's bigger 3.0 brother has a much higher final drive ratio. This makes it a better car above 90mph, but how often are you doing that on twisty country lanes? Drive both back to back and you might come away surprised by your findings.
If at all possible get a manual. It'll be less trouble at higher mileages and much more fun to drive. The only excuse for owning an automatic is the loss of a leg or if you live somewhere with huge traffic jams.
What I'm trying to say is that a BMW is a good package of all features. It is a balance of good handling, powerful brakes, willing engine and quality construction. You don't need a big engine to have fun, witness the 2002tii!

The Z3 that cost me 900 quid, superb car.
Conclusions
Don't buy a new BMW! Or even a recent one. You're spending more than you need to and you might end up compromising in ways you don't need to.
Get a cheap E30 or a Z3 for the summer and a three or five series touring for family use and bad weather duty. This way you get the best of both worlds and can still pay less than for one more recent car.
Don't worry about high miles, a well looked after BMW will easily cruise past 200k miles. But do insist on service history. And whilst you'd worry about depreciation if you did lots of miles in a newish car, it's far less of an issue in an older one.
My wife and I have five BMWs at the moment. To be honest four would do :-) They cost between 750 and 2100 GBP each. We've never had a breakdown or any nasty bills from any of them. Other than routine servicing and replacing worn out components only the E38 730i has needed work (poorly designed cooling system). They've done almost all the depreciating they're going to, two are rising in value. We get just a much driving pleasure as we would from something we'd spent 30,000 on but have older cars that look better and are much cheaper to own.
When I was young I used to buy progressively more expensive and newer BMWs as my means grew, newest I ever had was a year old. Then I bought an old 635CSi as a second car and realised that newer was not always better. I could have more fun buying older cars, and they didn't cost any more to maintain.