id go for the m5 mate just be ready for a big bill if it goes wrong remember that car was over 60k new which means the parts are still going to be that of a 60 grand car thats what put me off of getting one. i found the e46 m3 to small and far to common i must see 5 m3s every day how many e39 m5s do you see? Try and get an m5 with warranty if you can an 8k m5 could easily turn into a 14k car if your not carefull.
Also who are you getting insurance quotes from its was £250 more to insure an e39 m5 than my evo 8 340 mr?