I've been thinking about this, too. In order to gain efficiency, therefore some power, but keep it as light as the speed 400 setups, I'd look into a geared hacker B20, either 15L or 18L on 8-10 small cells like the 1100 Hecell nimhs, and a folding prop that draws about 15 amps max. The geared Astro 020 would also do nicely, but a little heavier. For really light weight, a Mega 16/15/7 direct drive on 8xHecells and a 9x5 or 9x6 prop should do well and eliminate the gearbox while keeping the power strong with the big prop. In any of these, I'd recommend the Phoenix 25 esc, just because Castle Creations esc's are my favorite. Look at www.megamotorsusa.com
for some good info. Search the ezone for threads using keywords like 'B20-15L' '16/15/7' etc, to find threads that discuss these systems. You might also ask the moderator to move this over to sailplanes, if you dont get the help you need in this forum. Good luck however you go!