£100 for the clucth, maybe £300 to fit it as the gearbox needs dropped to do it. I can't remember the price of the aerial from toyota, but im pretty sure it was over £60, you might be able to get one cheaper from tcb which are a toyota breakers. A second hand one might break again anyway, as the problem is due to a little plastic cog.
I have falken FK 452s on my celica, which are a Y rating. Don't see why you would need z?
You can sign up to either
celica club or
6gc for some more support. 6gc has a lot of american forum memembers, and celica club requires a subscription for the main parts of the forum, so not perfect. There is also toyota owners club though.
If i were buying one id check the normal things in a car, check the radiator fluids, if they are really nasty and running low theres a good chance it hasn't really been taken care of in general. Check all electrics work (windows, mirrors, sunroof) don't be alaramed that the sunroof (if it has one) stops half way when closing, for some reason its supposed to do that. Check that its had a cambelt change too. If you get a test drive bring it to a hill, get up a little speed and put it in fifth, if it nearly stalls/struggles then the clutch is fine. If your revs get higher and your speed doesn't, its slipping.
Im struggling to think of things celica specific. Other that then ABS and clutch on mine, i've had no real problems. ABS sensors are £120 quid each from toyota, and near impossible to get 2nd hand, so definately get one with working ABS.