1) Berkshires would work for me. Sept. can be very nice and even if it rains it normally isn't too cold.

2) How 'vintage' must a bike be in order to qualify? My 'vintage' bike is all new components mounted on a 1985 frame. But I did keep the headset and put fenders, DT shifters, etc. on it. ?

3) For an inaugural event, keep it simple and make the logistics easy. Ramp it up from there based upon rider interest.