Hi there,
Most common issues are VVT rattle so I'd look for it to have been changed if the car is over 80k miles or factor in the cost of replacement during your ownership. Factory turbo oil seals fail a lot on these cars so listen for funny noises when the turbo spools or excessive smoke at idle once the car has warmed up and sits for a bit.
I've seen the fuel tank baffles come loose in a good few mk2 3MPS's now which presents an annoying scraping noise when driving but other than that just your usual service history checks and rust underneath. A few older cars have a boot full of water due to water ingress through boot or tail light seals or from the spoiler fixings so check that also.
If modded you will want to know where the car was tuned and that it also has uprated HPFP internals fitted.
Think that's it!
