As mentioned,, why spend £600 or so, + £700 more (when doing head gasket you'll probably want to change a few other bits too along the way) when you can get a clean serviceable vehicle for about £200 or so more???? This way you pays the money and get a car you can instantly drive. How long has the head gasket been blown? how badly did it blow? was the car driven regardless and over heated? how long were the fluids mixing? all these questions you can never be sure of so you have to expect the worse in such cases.
No old car is ever going to be 100% mint as people won't spend the money on them. Yes you will find clean cars, but mint? no, not unless a hefty price tag is attached to it.
Mine is something you'd class as mint, where I have overhauled pretty much everything including engine sensors and every rubebr suspension bush, cooling system, new window seal rubbers etc just for the sake of doing it/prevention. Each service is a minimum Inspection I as per schedule + extras I like to have done at approx 4,000mile intervals. Yes it was costly, but the number of slight improvements have made my car exactly as it was when new. For example, many people are advertising Supra TT's as mint, do you think they'll have had all the engine sensors replaced, injectors stripped & cleaned with new seals, new valve stem seals, every suspension component replaced etc to snure the car is exactly as it would have been when new? NO!
Please don't kid yourself into thinking most cars are 'mint'.