Love em. Got a few wrecks in and around the shed that one day, time permitting, will make the ETZ 250 with 300 engine that seems to me to be the best bet.
The disc brake is if anything too powerful, certainly off road. The styling on the 251/301 models is a bit 'odd'. I wouldn't lose too much sleep over which one though, go for how it has been looked after - Supa 5, 250/300 or 251/301.
If you are looking at making it slightly more off road capable be aware though that the 251/301 swing arm won't accept an 18" rear wheel whereas the 250/300 will. 16" rear knobblies are hard to come by. The styling of the 251/301 is however a bit more off road oriented, but thats about all.
The MZ list will argue vociferously over whether premix or pump is best, and whether to spend £15 or £60 on a rear tyre, but they will offer useful advice on keeping one running.
Richard
Get an MZ, one of the best bikes for your Pound or Ilb out there.