Genghis's advice is good and thorough. I'd particularly endorse his recommendation to get a BMAA inspector to look at it and the paperwork.
The Xair is a good choice. But I do like the AX2000 a lot. I've owned both and have in the past used the AX for instruction.
The earlier AX3 is not as good as the AX2000 but they can be had very cheaply.
I'd also endorse what G said about the Rotax 582 two-stroke. Its a good engine and much better than the HKS for the reasons he gives. Dont listen to clubroom "experts" and their scare stories about poor reliability of two-strokes. They need to be looked after and fed properly thats all.
Its true that the HKS sips fuel compared to the 582, but for average use that really shouldn't be a problem. And yes, spares for the HKS are difficult to get these days..
Edited to add: if you look on AFORS you'll see several AX2000s with HKS engines for sale and very few with 582s. But a check on G-INFO shows that there are many more with 582s on the register than with HKSs. There's a reason for that disparity.
Last edited by Heston; 21st Jan 2016 at 16:56.