Buy the best you can afford! Don't neccasarily settle for a cheap one as you will probably have to spend on it. Mine is a 2003 ( the final year the e39 was built) and it is £180 to tax for 6 months!!!
Front bushes are common weak spots as the engine is very heavy, but relatively cheap to replace. My advice, and it's what a bmw agent said to me once, is; if you can afford to buy one of these, you have to have pretty deep pockets to maintain and run one, .
( but then cosworth ownership is similar)
Loads of fun, still gets attention but only from die hard car fans and not the usual scroats!

Very reliable, providing it has been maintained properly, and very practical too!

Do it!!

I have been thinking of selling mine, but as already said on here, I can't find anything to compete.

Ask many bmw owners and they all say the e39 m5 is the best car bmw ever made!