i picked up an e38 3.5l on a 98-plate for £3k here in UK. before getting this i was always a merc driver, but the 7-series is doing a good job of changing my mind. like another poster has said, the car *must* have a service history. apart from that, use your judgement about the previous owner to decide whether that car has had everything it needs at the right time. the V8 can be comically frugal for such a big motor - but if you boot it around alll the time you will need your own oil well. from what i have heard, the 728i is a good car, especially post-2000.
parts prices vary tremendously. some are very expensive. the e38 suffers from a few model-specific faults, one being that the dashboard display doesn't ... display. £500 plus fitting. the suspension bushings go at the front, leading to alarming 'wobble' at certain speeds and under braking. the bushes are cheap, garage labour is generally not. tyres are expensive.
but they're lovely cars; on a recent trip to the continent, mine cruised happily all day at 100+mph, and returned an average over 3,000 miles of
25mpg.
good luck.