BMW certified diagnostic tech here. The 323 is pretty gutless, go for a 325 or better yet the 330. When you test drive it, at low speed, stab the brake pedal. If you feel a thump, it'll need control arm bushings. They are a wear item and are not too terribly expensive to replace. At dealer price they are $155 a set or so. It really helps to have a lift to replace them. Other things to look for in the 3 car is the rear lower control arms. Tow truck drivers will hook them and bend them. Roll down all the windows as they tend to have regulator problems.
With a 5 car, the Thrust rod bushings (Same diagnosis technique) are a wear item. Much more involved to replace, you need a press to get bushings out and back in. The rear ball joints wear too. Get it on a lift and from under the car grab the bottom of the tire and move in and out. If it's worn, you'll see it move. That infamous BMW handling doesn't come cheap unfortunately. I'm partial to the 540 because of the power, but if you get a 530 it'll be decent on power and much better on gas. The 525 is pretty gutless and much less nice inside. (Plastic trim as opposed to wood, pleather as opposed to real leather etc). The cup holders in the 5 series are absolutely WORTHLESS!!!!! Don't even bother to try and use them. They will just break. (And if they were $5 it'd be too much.) Search the web for the good additional one that goes in the cubby in the center console. With all this said, If I had the money I'd have a E39 M5. But I can't afford to go to jail right now so I'm sticking with the 4runner. I also recommend a manual tranny over an auto as they last longer. BMW's are also a bit more to insure. Take that into account when buying. They are extremely fun cars to drive and you must use cruise a lot or watch the speedo like a hawk, cause they are just as smooth at 100+ as they are at 60. If you have any specific questions don't hesitate to ask.
As for who works on it, keep this in mind. I've got a Z3 right now that the lady had an aftermarket shop in Charlotte work on. They have F'd up this car so bad I've had it a month trying to un-f it. They've added additional ground wires that are supposed to be grounded by the engine controller and God knows what else. The bill, well it's probably already more than the car is worth. Choose your mechanic wisely. They may claim to know how to work on anything, but they rarely do when it gets really technical. A BMW ain't your average Chevy.