Love my M5. Bought it three years ago with 45,000 miles. Put about 5,000 miles per year on it. No issues, except for a rear wheel speed sensor, the aforementioned FSU blower resistor, and a faulty fuel vapor purge valve. All easily DIY. Purge valve did require removing the plenum.
If you are looking at a 2003 model, sub 50,000 miles for 18,500 that is clean, accident free, and unmodified, buy it immediately or it will be gone within 24-48 hours of being listed for sale. I'd say a fair private sale number would be $25,000 to $28,000 depending on colors, options, condition (new tires, clutch, etc.
At less than 20k, a specialist out of Ohio (Enthusiast Auto Group @
www.enthusiastauto.com ), will have cash in hand faster than you can get arrange a PPI. They will bring the car to their dealership, spit and polish every nick and cranny, and then re-list it in the mid to high thirties.
I lost several cars to them when I was shopping. You have to be quick on a 2002, 2003 model that has low miles and a clean history. EAG won't buy 2000-2001 models.
Clean low mile E39 M5s have been slowly appreciating the last few years.....not air-cooled 911 appreciation, but they are slowly moving up as they become more rare.
Good luck! I'm sure you will enjoy it if you buy a good one. Like a Porsche, buy the best you can afford, avoid the neglected or abused, and you should be fine.
Edit- All the help you could ever need can be found at M5board.com. Great DIY how-to's and a tremendous knowledge base for these cars.