No trials bike that I have ever ridden is "hard hitting" off idle. It defeats the purpose. You need to be able to roll on the throttle without breaking traction. At sea level the 280 will be pretty peppy, but trials motors are very linear. If you don't wind it up with the clutch in then dump it, it will be very smooth. Clutch and throttle control are key to trials. I don't think shimming the jug is a good idea...
Yes it would be a great starter bike. The parts are quite available, and there are lots of them around. As to price. I was considering a similar 07 300Pro for $4500, in like new condition, with no scratches on the skid plate.
As for what to look for. The suspension linkages in the back wear out. They are not cheap. If the rebound is gone in the rear shock. Not cheap. Shifter can bust, but if it does shift, you're probably ok. Shifting will feel crappy, compared to just about anything else you have ever ridden. They are just like that. These are astonishingly durable little machines. Unless it has been completely abused, and the oil never changed, it will probably serve you well. Rev it up, then cut the throttle. If you can hear piston clatter, it will need a top end, though some say run it till it blows, as these are generally low RPM machines, and it won't be catastrophic.
Then ride it till it dies.
ATGATT: When you fall off your motorcycle, you will be wearing what you were when you got on your motorcycle. Dress accordingly.