I would say if the seller is providing you with 3 months warranty theres a pretty good chance the motor is fine, its not likely if the engine is muller'd it has a 3 month time'd bomb till it blows!
As you know Rob, i pick'd up a low mileage motor for mine for a bargain price ;) The seller let me do all checks, apart from not seeing it running its a perfect :)
Personally i'd just get the motor, and i would not settle on paying him what the ad price is, haggle... but thats my middle name ;)
Becareful!
If you can get a engineers workshop manual. I have one of the head rebuild which i paid to get translated.. cost me a fortune! (30 pages odd)
Head set, which will inc. every gasket. seal on the head is around £100
Head bolts, £70 for the set
cambelt, £30
Tensioners, £100-150 for the lot (1 tensioner, 2 idlers)
Piston rings, no idea...
PM Chris S for prices on other parts...
All i will say is, DO NOT take apart the cam wheels. its very easy to take them apart as dismantling them from one of the castings that goes on the head can be tricky, and it may appear that the only way is to loosen the bolts on the camwheels... u have to a little to get to the 2 10mm bolts holding that casting to the head...
If the camwheels come apart, your better off buying brand new assembled ones, around £100each!!
VVTi is a pain the *****!