Maybe you need to ask this question on the myE28 board. I would have to be a little biased and say that an E28 M5 would be a tough family car as far as reliabilty goes. I could be wrong. A very capable car but expensive(parts and paid labor) to maintain as they age. Also more of a 9/10th driving car. If I choose an M5 once again, I would have to go borrow Jr's M5. The E34 M5 body style showed BMW that if you make it more than one color, They will come.If you must get an M5 and for all BMWs, get it an inspected and get the best example that your budget will allow. Check for good maintenance records. LOL in your search.
Willo