You need to use the theory of if there is no proof of its the cam chain issue being sorted (invoice of either new engine or the updated version of the cam chain tensioner) then it needs doing for peace of mind, mine had already got a new engine in it and its been as reliable as a wood burning stove.
If you don't know it's been sorted, use that to beat the price down, and/or get the garage to fit the new tensioner before collection.
My car uses almost no oil at all, i check my oil level each week and I have only topped it up 2 or 3 times since buying the car over a year ago now.
I'd definitely recommend mk6 gti. get a good car, Look after it and it will look after you.