Joe,
I've only had my 'Lucy' for a couple of months but have been very impressed by it. Mine is an eight seater with 4WD and a 2.2 TDi. It's got twin aircon systems, alloys, smoked glass, curtains on all windows, twin sunroofs, e/h mirrors and so on. And with only 35K genuine miles on the clock I reckon a bargain too at £4500.
I'm particularly impressed by the handling.....very car-like. It was considered the best handling people carrier of its day because of the engine being mounted amidship and underneath the floor plus of course the 4WD helps as well.
As for things to look out for? There doesn't seem to be anything drastically problematic with them but if your friends join the Estima UK Forum (link below) they can get good, sound advice from more clued up people than I.
UK Toyota Estima Forum