Like the others have said...
Head gasket - not normally a problem on the KV6, however the thermostat can go (i don't know if you can look into the bottom of the 'V' without taking off the plastic cover but the water / coolant mix lies there)
Timing belts - should already be done but just make sure (will cost in the region of £500 if they haven't)
VIS motors - Normally just a broken solder joint so can be fixed if you have the time. Some of us have fitted 'oil catch tanks' as damage limitation
Plenum - Can also fail (rattles and car is 'down on power'). No real 'quick fix' yet... Just a new one at £500.
Clutch / Slave - Can go but theres a 'Sheddist kit' to uprate it. Either £80 or £180 (can't remember which)
...am sure there are a few other things but that's most of the stuff summarised.