E39 M5 is one of hte most smoothest car( with that much power) I have ever driven. I have owned mine since 2005 (bought a 2002 with 55K miles) and had very little issues. Currently the car has 77K and seems to run better than ever.
I would recomend driving the cars, before you make your final decision.
If simply driving the cars was the answer, that would be it. But it is not, a smart buyer looks at the total cost of ownership in time and money.
There is no question the E39 M5 is one of the best sports sedans ever built. And it is possible to go for a while with very little issues. But there will come a time when the coolant system
will fail entirely in an E39. And the pixels
will start going away on the dash, and the door/lock actuators
will start failing here and there, and the power seat cable issue
will arise, and the lower lumbar support
will stop holding air, and so on. Then there is the dreaded carbon buildup in the secondaries which you fix for $8 grand, or buy a powerchip for $1 grand so you can pass emissions. And even if you do the work yourself, the parts are expensive.
After driving E39 V8 BMW's for over 100,000 miles, personally owning two, and also taking E39 540's and M5's in on trade at our dealership, and after watching all the comments on M5board.com it is possible I may have a little insight on the E39. So I posed all the questions above to cut to the chase and help him determine what he needs. Also, I been in the car business long enough to know that sometimes when people say their car has been trouble-free, and then I see their stack of service history receipts - there is a disconnect with their perception of what is trouble-free!
I figured I owed it to this guy to be quite frank on his choices having personally owned/driven both products. Everyone has a budget, and an M5 will cost far, far more to operate over time that a good solid Mitsubishi EVO. And the EVO is no slouch! That is a terrific car.
Put in my terms, maintenance cost is the reason I am going to get an NSX over a Ferrari. I don't want to have to pay $6,000 to $15,000 every 3 to 5 years for preventative Ferrari maintenance. Even though it is old, the NSX wins hands down in that area. I sell 150 to 200 Honda automobiles every year and have come to genuinely appreciate their solid engineering and build quality.
I would be interested in seeing the OP's answers to all my questions, but based on the responses I saw to the few that caught his eye, there is no question the EVO would be the best fit for him.