In Alt do a check on G-INFO and trace G-OTAL, she's the one with the Rotax engine. The chap who owns it is the UK Rotax dealer so should be easy to trace. I have no idea if it's for sale or not as it has quite a lot of non-standard instrumentation which has been fitted to develop the installation of the Rotax I would assume. I'm sure he'd flog you a Rotax and installation kit if you can find an engineless airframe!
PS We used to run out of steam flying out of Popham when two up. I was 14 stone at the time and the instructor was about 15. If your body mass isn't in this league your problems would also be smaller.
The best advice is see if you can persuade an ARV owner to take you for a whizz about for a reasonable contribution to the costs. That's the best way to make up your mind.
PS I did a chunk of my PPL on G-OTAL