when garages talk of a full rebuild they usually mean "stitch the sod up and put a second hand head on it"
Truth is, your pistons are very likley to be fine if its only slightly out so all you are looking at is possibly a few bent valves and maybe not even that.
A cam belt can't slip, it can only "skip" a tooth or teeth. If its skipping a tooth your cam belt tensioner is toast or the cam belt is shagged this will be taken care of with the new belt kit. which you should insist includes tensioner and water pump. (50 quid for the lot)
My advice, get a decent mech near you to re-time it as is and fit the new kit. It may be OK if the timing is only 2 teeth out. and only then will you know if you got away with it.
It deffo wont start if its 2 teeth out as the valves are not closing to allow compression to develop. It may not even have bent a valve but you won't know until you get a good mech on the job. If the valves are shagged get a recon head or a head out of a crashed one and you are in business. changing a head is deffo not a rebuild so dont be taken in mate. This is about 5 hours work for a decent mechanic.
worst case, if its jumped 3 or more teeth then the valves will be shagged and possibly broken which could well have split a piston and that is a full rebuild or an engine
Ontor