RE: Maximum altitude??
There is not much choice of motor kv and prop diameter/pitch, anyway, so you don't have to think that hard about optimum drive.
There are only a few big folding props, those I know of are the Graupner CFRP 20x12 (and maybe 17x10, [link=http://www.graupner.de/en/products/a19e1365-97ab-4565-bb27-a7b06a1c1fea/productcategory.aspx]here[/link]) and the aeronaut CAMcarbon 18.5x8/10/12/14/15 ([link=http://www.aero-naut.de/en/products/aircraft/airplane-accessories/propellers/camcarbon.html]here[/link]).
I found one really low-kv motor ([link=http://www.modelmotors.cz/index.php?page=61&product=5360&serie=20&line=GOLD]AXI[/link]) which is even made for a 10s LiPo. They recommend their special 30x20E propeller but it's not a folding prop. It might still be good for this project though. I didn't check what matches.
But may I remind of further problems that already have been mentioned. The long runtime of the drive means there will be some heat to be dissipated from the battery. Maybe the 10 cells should be installed separately and not as a pack to get rid of the heat. Maybe it's not a problem because half an hour runtime means only a 2C load, but it has to be checked. As well has to be checked if the battery has to be warmed or at least insulated during motor-off flight (descent in the cold air at 15k ft).
Due to these problems I would use a radio with telemetry (besides the UAV control devices) because an ESC with integrated telemetry would save weight and hassle. Add temperature sensors for motor and battery and you have the drive under full control. Unfortunately, Multiplex announced those ESCs more than one year ago but still doesn't ship them, so there seems to be a problem with them. Maybe it's a problem with the switching BEC, though.
And an idea regarding flying the thing: The ESCs have a break that stops windmilling so the prop blades can fold by the airflow and drag is reduced. You may want to have drag, though, for landing approach control. If the throttle stick is slightly advanced from the off/break position, the motor turns slowly but the blades unfold and windmilling makes for an airbrake effect. Saves separate spoilers but requires some charge left in the battery for landing (which is good for stretching the approach as well.) But it has to be checked if the drive would give enough braking effect at all.
And yet another flying thing: A R/E plane may be tricky to land in gusty wind because there's no separate yaw and roll control, even less crossed controls. And worse, a high-aspect-ratio wing makes for big yaw and roll inertia. So I would just practise in a simulator, even if the plane had ailerons, or are you used to gliders?