I am a big proponent of lighter/less power is better with trials bikes. If you are not riding beyond intermediate, and you weigh 120 pounds, I see no reason to have a 280. Trials is much more about what you as a rider are doing on your bike versus what bike you are riding. You can do a lot more on a bike you are comfortable with versus one you are not.
I have never ridden a 80 or a 125 so no help there. I did put a novice girlfriend of mine on a 97 TXT200 GasGas and she loved it. I would recommend a 200. They are real mellow in stock form and with an extra base gasket they start easier and are even more forgiving.
If you are comfortable on the 80, go for it.
There is a 200cc Sherco on the UMTA page: http://umta.org/used-bikes/