Agreed, I'd love an E39 M5 and they're within striking distance budget-wise to acquire... Not sure about maintenance though as they are a complicated beast (not that the GTR isn't, but when you have self levelling dampers, about 6 oxygen /lambda sensors, etc there are many things that could fail)
The other "problem" is that because they are such a comfy car, many have done high miles. Looking for an E39 M5, it's rare that a well looked after one comes up with less than, say, 120,000km on it...