Capable mileage..? Seen plenty on 250k +, also seen quite a few that have given up the ghost at a low mileage, look after it, and it will look after you...
What to look for, well E30's biggest enemy is rust, though that said, convertibles and touring don't suffer anywhere near as badly as they're saloon counterparts.
Places to check for rust are, scuttle panel below windscreen, under fusebox, and same area on opposite side under battery, lower front wings especially if fitted with bodykit/mudflaps, rear arches, be sure to look and feel inside the arch too, around rear taillights, number plate lights, and behind number plate, also check all areas behind the trim inside the boot.
325i also has a good appetite for cracking cylinder heads, look closely for any signs of mayo under the oilcap, or on the dipstick, if the car has been sat for a while, this could just be condensation, but this would clear after a run if it's not mixing, also check coolant for oil, and general condition, it should be a tidy antifreeze colour, and not rusty coloured.
Brakes and suspension are both easy and cheap to replace, but can be a good bargaining point!
Good luck!!