The main difference between the 120 and 140 are the flaps on the 140 (which don't do a lot anyway). Both aircraft originally had fabric covered wings, although the 140a has metal wings and a single strut (like a 150). Several of the fabric wings have been "metallized".
The 120s (but not the 140s) were originally allowed onto the PFA permit scheme. I believe now most factory builts including Jodels, Condors etc are forced onto private cat. If you want one on a permit, I think you will need to find one already on it.
The 120 will outperform a 150 in almost every respect on less power (and fuel) and is a very practical little aeroplane.
Originally they could be bought without brakes, electrics etc, but most of them have been updated.
A lot of them have the wheel extensions which put more weight on the tailwheel. I've never found any need for them personally. They are very easy to land for a tailwheel, though it takes a delicate touch to avoid a small bounce.
I've had a share in one for over 13 years now and have little to criticise. Let me know if I can help with any specific queries.