Just with the base price differences of the cars, i have to assume you're looking at a used 135i.
Firstly, the 1 is very easily modified, so you're claimed "never tracked" used 135 still might have been running a chip, and the HPFP issues are pretty well-documented. Also, on the street it feels great, but at the limit its weight and body roll REALLY show. All that said though, i thought it was really easy to adapt to driving it. Bob Clark has a strong opinion that the car is not classed appropriately in D Stock, and since it's a heavy high-powered car, logic would think that with Street Tires it would be a bit more out-classed.
All that said, it's a ton of fun. I think it'd be a blast in STU, probably moreso in BSP where you could play in the 400 torque realm and mods to handle it, but that's throwing a lot of money at an already expensive car, at which point it would make more sense to buy a 1m coupe which is basically a ready-made STU built 135i (but with a wide-body kit and 30 more torque), if you can get on the list and be willing to wait until june.