I would personally say no.
Lets face it, its a 125 and as such needs the tits revved off it to get to 100. The powerband is also tiny. For £700 i would expect it to be old and knackered. Assuming it is a 2-stroke it would need a ridiculous amount of maintenance and IMO you will continually be throwing good money after bad.
If its a 4-stroke, then its better reliability wise....but your way down on power!
I'd say save a bit more cash and get something with a real engine
