Firstly, thanks everyone for their responses and input.
The Rotax in a J430 is a good option in most regards, except the price. The quote I have puts the engine at pretty close to $40 000, which has real potential to overcapitalise the airframe (IMO).
The UL power and D motor, look like an option, but similar to the Rotax they seem pretty expensive.
Yenn, I am interested in your experience with the O320, in particular what percentage of power are you pulling for 140Kts and 30Lph?
I had thought an option was to look for a used O320 with 1000 or so left to run and then I can upgrade to an O360 if I ever wanted to. (What are the chances of picking up a used O320 for around $12000)
Skippydiesel, The ATEC Zephyer looks to be only available as a two seater, I am looking at 4 seat options at the moment for the reasons below.
The 4 seat requirement is mainly so that I can carry camping gear etc for longer trips, plus I'm not a small person and my flying partner is similarly built so most two seaters don't give us the range we would like when we add even basic gear.
We have done a few long trips in a J170, but range hurts us for some areas, most recently Mitchell Plateau where we had to bail because we couldn't fill the tanks to capacity.
The Cougar 2 does looks like a good option, thanks Ray. You can probably look forward to a few more emails picking your brain about it!
Anyone have any input on running costs associated with the O320 in an experimental airframe given that I intend to do my own maintenance? I suppose that any spares required would generally be certified and therefore expensive.