i wouldn't do it.
at the end ouf the day although 205gti's were a great car at the time, you'll find it hard to come across one now which isn't suffering form its french contruction.
i just broke up my saxo trackday car which was only 8 years only and the french build quality had meant the only origonal part on it was the shell, and thats a whole gernaration on from the 205!
engines are not sophiticated in any way! week compared to any vag lump!
Very simple engine management and will not return both a smooth idle and resonable fuel economy!
At that milage expect it to burn oil, everything will be well worn in!
Rear beam trailing arm bearings will be dead with that miliage, your lucky if they last 60k most the time
Also the seller does sound dodgy as hell, you can tell he clearly thinks hes onto a great deal!
I've seen much lower milage 1.9's for less money
the 1.9 is not less tunable neither engine is a great engine to tune as the design is donkys old, the 1.9 only make 130bhp with a mentaly wild cam as standard (lumpy idle and all!)
A 1.9 with a 1.6 gearbox is pretty quick