I've just spent two years restoring an e28 528i on a D plate. So I know a little...first thing is under pinnings, bushes etc. There are two massive bushes called rear subframe bushes, these are about the size of a coke can, just a little shorter. These go after about 100,000 miles, and are about £70ish a side to replace at a specialist. You can tell if they're worn because there will be a clunck from the rear of the car as you pull away. If anything else is lose, eg steering, then this with be the control arms. There are two for each front wheel.
M30 engined cars, eg 2.5, 2.8, 3.5, are prone to cylinder head cracks, so if you see oil and water mixing and you're not prepared to do it yourself then walk away. Boots tend to fill with water, so check for holes, if you don't find any and it's wet, then it's light cluster seals and the boot lid seal.
Apart from that make sure all electrics work and that she has been regularly serviced. If it's an auto, check the oil in box, if it's black and smells burnt then change it straight away, if it's red leave it.
If you have any questions then post a msg. I think the e28 is a fantastic car, it handles well and if it has a good engine can be fun to drive, there're mad for them in America. I must admit mine is not standard, I've put a hot Cam in it, ported and polished the head, modified the valves, put on a sports exhaust, and had her resprayed in the original colour of diamond black, mmmm nice, and to treat her I'm taking her round the Nurburgring in May....! I'd still like an e28 Alpina B10 though, perhaps later in the year...