To me a top end rebuild which I would expect to include re-seating of the valves, skimming the head if required, replacement of the valve sleeves, maybe a new cam,and generally a good clean up of all the component parts. However that's just my interpretation, others might be different so you need to understand exactly whats been done and why it's been done. If I got a major job like this done to my car i'd keep the receipts so they should all be in the history of the car.
Depending upon what you want out of the car the later 250 turbo's are potentially more desirable. They have bigger front brakes, LSD gearbox with strengthened 1st & 2nd gears & driveshafts, an oil cooler and of course a bigger turbo. They also might have the optional M030 suspension It's definately the way to go if you are planning to incorporate mods or track the car. If you're planning to be one of those rare people who want to keep the car standard (or even lightly modified) then a 220 turbo will be perfectly satisfactory for you - plenty of pace and performance and the same superb on-road handling. You could argue that the bells and whistles you get on the 250 turbo cars are only really beneficial for the track - especially the M030 suspension (lower ride height and harder ride). An LSD is not always the holy grail alot of people advocate - they can induce oversteer in certain conditions where an open diff would just allow one wheel to spin freely thereby keeping the car on track with the three remaining wheels that still have grip. This can catch out the enthusiatic driver that hasn't quite got the experience or skill to handle the LSD.
In terms of the car you've seen, well all you can do is go and view the car. Make sure it's got a good history (remember the turbos have a 6k mile/6 month service interval instead of the 12k mile/12 month service schedule of the N/A cars), the cars condition is consistent with the history you've got (pay special attention to the belts replacement) and the overall condition of the car seems in order. Look for any smoking on start up or under accellaration which could be an indication of a tired turbo - but you should expect to get at least a good 150k miles out of a turbo.
Happy hunting.
