As much as I concur that this is probably the best 'buying guide' article ever written about the E34 M5, I must add that had I read this article before I bought mine, I probably wouldn't have bought one!!! It would have scared me off completely!
On a serious note - I did a small bit of research before buying mine, but the most important thing that I did was to take someone else along with me, just to make sure that I had an extra pair of eyes to spot for anything that I might have missed. You know what it's like when you have a wad of cash in your pocket, you can let your heart rule your head and common sense doesn't always prevail. Originally (back in 2001) I wanted an E36 M3 Evo Saloon, but that is another story...(BTW, I am glad that I didn't).
In 9 years of ownership, I had just short of 8 years of completely trouble-free motoring and in that time all I had to put in was petrol/oil etc. This also includes being exported to Canada (2005-07) going through two winters of -30 deg C, and three years in Germany where she regularly stretched her legs legally, before finally returning back to the UK this summer.
However, in the past 14 months she has had a recon gearbox/diff and a complete sls/edc delete because one of the rear shockers was leaking and it failed the MOT. The cynic in me says that time has caught up with her but I put down most of these problems down to lack of use - about two years ago she stopped being my daily driver and was just used at weekends and that was when the problems started. Now that the suspension is finally sorted, it is being put back into regular (everyday) use, well as soon as this snow disappears!
The thing is though I really don't mind, because I will find the money, whatever it costs, whenever I need to, to keep her on the road. IMHO, I had more than my monies worth back in the pleasure that I have had since I have owned her and I can't bear to part with her.
The bottom line is - look after them, service and maintain them properly and they will go on and on and on........