Slip in all forward gears will be the front clutch, if you have slip in reverse as well that's either the rear band if you've also lost engine braking in "1", or the rear clutch. In most cases you either lose forward or reverse drive not both, but if you have and it does kickdown I'd check the fluid level first to be certain that was correct, if it is, then in the absence of any more detailed information I'd say you're looking at having the box out rather than being able to do anything in-situ, but without the special installers you won't be able to replace the piston seals, which are likely to be the underlying cause of the problems.
£1500 for remove/overhaul/refit is probably a bit dear, but with a few unexpected extras along the way (like snapped exhaust manifold studs as an example) it's easy for the price to creep up.
A "torque limiter" is a new one on me, if you mean the torque converter, I think it's very unlikey that's the problem if it drives as normal, but slips under high load conditions.
Secondhand boxes are a lottery as they don't respond well to long periods of storage, and £150 isn't cheap either.