Focus ST's have problems with cylinder liners cracking - as well as lots of common little faults. They are thirstier than the Ed30 too.
81k is nothing for a TFSI - loads of 100k+ examples out there, even seen a 200k example with DSG a while back.
Look for service history, and general care & maintenance. i'd recommend you replace the cam follower, diverter valve and PCV when you pick it up - particularly the cam follower (do a search on here for more info)