My 2p's worth.............. personally I wouldn't bother with the 540 unless you want to spend a lifetime at the pumps. Also the V8 models had a recirculating ball steering set up and although I haven't personally driven one, am told the rack and pinion models (528 and below) are better.

The early V8's had nikasil liner issues the same as the 6 pots before 2000 so check with BMW UK about engine changes etc if you go for a pre 2000 car.

I had a 528i SE and they are great cars, pound for pound very hard to beat as a used buy in this category, even more so with prices being dirt cheap. Very reliable on the whole, some problems with worn bushes on the front which can give a high speed ''shimy'' which can be misdianosed as warped discs etc.

530d's are AWESOME motors but IMO you won't get a nice tidy example for your budget.
Stories about injectors, fuel pumps, turbos etc are reported indeed buy IMO again think they are exagerated.

For you budget, I'd go for either a 528/ 530i sport model with sensible miles and some extras. Will give you champagne motoring for lemonade money