I think that would be a good idea. I'm sure it's a great amp, although I've never played one so can't comment. The questions I'd be asking are- How old are the KT88's and when was it last serviced and by whom?
If a new pair of KT's are around £70, a service around £30 (not too sure about this figure), then see if he'll go for £700 and you're covered if something goes west. If it's just been done with all new valves and receipts then try to go £750, otherwise you may as well buy a new one.
The KT's I've shown are Gold Lion, which are more pricey. Matamp only fit these now after QC issues with JJ's. I've still got JJ's in my GT200, they're doing fine after 3 years and a pair is £56. I sometimes think there's a bit of luck involved when it comes to getting a good set of valves.
Hope all goes well, they do look a lovely head.