Which years/generation?  Realize whichever you are interested in, it will be a labor of love.  See video below, but you will need to learn as much as you can about how those BMW cars work and be armed with knowledge  (and all the nuisances in those systems), do as much of the work yourself as you can, almost to the point of being a pseudo-expert, and know where to get parts, and still be prepared to spend money.  If you are not prepared for a commiting relationship with the BMW (will have its ups and downs), I would pass.  It’s not for the faint of heart.

https://m.youtube.com/watch?v=VdVsbLQvlMM

They are money pits as they age (almost irrelative to their mileage) because of highly used cheap plastic in them. So, the best advice is not to buy them. 

I owned a 2007 BMW X5, from 2016-2020.  From 160K miles to 200K miles. And owning one taught me a lot about cars and how different makes engineer their vehicles.

They are amazing vehicles.  And they are relatively reliable. But, and a huge but… they are also expensive to maintain.

Many things may need repair or maintenance like any other car.  It just cost so much more on a BMW.

FWIW, I spent more money on those 4 years of BMW ownership on repairs and maintenance than I did on my 1999 Honda Accord in 21 years.

So. You looking for a restoration type? Or daily driven? Either will take a lot of effort especially as older BMW's get more "rare" throughout the ages. You have to have passion and not just toss it off to get it fixed Everytime it breaks down.

What year are you specifically looking for? The v8 5 series engines are a little low on the power scale for being a V8 and they are much akin to the crown Vic in that regard, being a boat to drive really albeit smaller than a crown Vic.

I wouldn't be so concerned with the plastics but again it depends on if you just want the thing to drive or not. @Yaser. Yes and No. Cosmetic plastics will break relatively easy compared to other older cars. If you're talking about engine plastics like connectors and all that that is more of a common thing between all older cars in general. My e30 was sitting for 8 years and the plastics all are still okay. Many in the community say the same as long as you ain't trying to rip out the dash every time you work on the car. So a bit of a misconception I would say. 

@Cars, what are years are you looking for man? Anything past the 4th gen are not worth anything at the moment, and what's your definition of good/good enough? It do be a BMW so it will be a nice example of german engineering, mine came with a full radio/telephone setup, power heated driver seats and power windows from 1987. That's crazy to me...

If you are looking for 1st to 4th gen Good luck with even finding parts at a good price. You'll find em but no way I'm paying $600 for an OE front valence or $400 for an "un-cracked dash" prices be ridiculous and the market is not reasonable for these cars. 4th gen will be easier since it goes to 2003 but even still. If it's reliability you are looking for You're looking in the wrong place. If it's for the cool factor then you be in the right direction, well for me I'm bias haha. Not to say you can't daily drive an older bmw, as @daywalker has mentioned, there are a lot of tidbits that you will need to know to keep the car "reliable" and practically have to know the ins and outs. For my case, always keep an extra fuel relay, odo and SI board is always broken, dash is impossible to find uncracked, fuel tank needs to be constantly cleaned up, the AC system is a pain in the butt to keep working, I just ended up getting rid of the unit all together and replaced it with 2 amps for that sweet sound system and I live in California so who needs it lol. Forgot to mention that ripping out the AC took me a day to do as there are no instructions anywhere so you either know or have to figure it out. 

IF YOU ARE INTO these types of cars, be sure you got the grit to continue with the project. It be a waste if you bought it, ran it for a day and then figured that you are now stuck with a pile of barely working parts. At least for me I consider my project e30 to be a never ending one and I am content with the fact that it will always be a historical car that I can wrench on and modify anytime. If you don't have this type of determination for a project. And are just going to toss it to a mechanic anytime, then there is no real reason to get it. I'm not gatekeeping. I'm saying logically it is a project car if it is an older BMW. So time, patience and money are needed. I would steer clear otherwise. There are plenty of other cool older cars that don't require as much maintenance and are still in the older category, few I like as well be the Toyota crown/Cresta/Cressida, Nissan 240(any in this generation), Volvo 240DL, original Honda hatches, VW rabbits. But hey at the end of the day, these be my 2 cents. If you are willing then the money and time is yours to spend haha.

Endless moneypit

Never buy a newer BMW with the V8. BMWs (Like the 5, 6, and 7 series, X5) from the early 2000s and up use the N62 and N63 V8 engines which were rolling piles of junk, with excessive oil consumption (about a quart of oil burnt every 750 miles), timing chain guide issues, valve seals which is BIG BUCKS since the engine has to come out and the heads have to come off, and the turbos going out which is also big bucks. The best BMWs are ones with the inline 6 excluding BMWs with the N54 engine since those were problematic.