Things to look for:

1) rear main seals, if you look at the underside of the car it will be oily and should be spots of oil.I only discovered this after i bought mine. Costly of you go to a dealer Parts are cheap but labour isnt as gearbox comes out.

2) Turbo oil return line, Again seal that has gone - cheap to get sorted.

3)Air-con, Look in the engine bay to see whether dye has been added to check for leaks. Let car idle and turn air con on. it should be ice cold and compressor should not cycle.

4)have a good look at the rear tyres as the 850's have passive steering in the rear wheels.If the tracking is out then you will go through rear tyres.

5)check upper engine mount as the can go, Part will cost about £10 and hour or so of your time.

Also remember to check things like brakes as 850's go through them quite quickly especially if auto.

Above is basically what ive discovered about mine.Ive got the turbo and love it to bits,fast,safe,comfortable,all mod cons and looks sweat.

The 850 may have these little problems but are reliable and if you look after them will go on forever.