Since it will be a glider like layout but with an electric propulsion system a lot of the "limits" of a glider are not valid for the discussion.

IflyPATTERN, a good starting point is to chart and study the designs of a lot of the bigger compeitition sailplanes out on the market for wing area, weight, the resulting wing loading and the airfoils used.

Note that these designs are set to fly well at a cruise speed of between a speed range of around 15'sh to 30'ish mph. Your use of electric power should be able to extend that sort of speed range to some extent. But I'd suggest that you consider the electric system as more for climbing than for cruising at high speeds unless you find you need it to return to the launch point.

Note that sailplane fliers have learned to work WITH the wind of the day so as to avoid any return issues. The tend to hunt for their thermals by flying upwind and only allow the models to drift downwind while circling in the lift which will allow them the "fuel" (altitude) to speed up and get back to the launch point or carry on back upwind to hunt for another thermal. You'll want to plan on flying your UAV in a similar manner by noting the various wind speeds in each layer you pass through on the way up so you can navigate in a manner to have yourself be "upwind" and glide home instead of being caught "downwind" and have to power back or go hunting for the landing spot. Even a powered sailplane is at the mercy of the winds due to the slower flying speed.

There's no reason that a more or less 3 meter glider like craft such as you're looking at will not reach up to around 15K feet. But much more than that and you're going to find that regular props and model designs will begin to suffer more and more as the air thins. But all is hardly lost. Full size sailplanes and certainly UAV's have gone much higher. Even the record for model aircraft altitude is up around 18K feet as I recall. But if you're looking at getting up a lot higher and even touch on the stratosphere then you'll need to go with increasingly bigger and fancier options which stretch the link to the bigger contest sailplane formats.

I think you're still on the right track with a large glider like craft. The long higher aspect wings will give you the sort of efficiency and lift you want for the sort of mission you describe. The real trick will be juggling the drive system and folding prop. As for the prop I'd suggest a way to selectively allow it to lock in the extended position or fold. When folded the craft should make a decently respectable glider. But for descending a large freewheeling prop makes a superb speed limiting dive brake. If you were to set up an option for allow it to lock to stay out you could power up to extend the folded blades, operate the lock then power back off for the braking mode. To go back to folding mode simply power up for a moment to take the pressure off the locking pin and unlock then power off.

Structurally keep in mind that a P-38 style twin boom may provide the ideal place for a large pusher prop but it does give the designer some structural headaches. So often folks want to stick small section carbon tubes or similar on such designs becuase they look sleek and sexy. But the tail booms must be stiff enough to withstand flexing that significantly alters the horizontal tail angle to the wing. Note that I said "stiff" and not "strong". There's a very important difference between the two. In concert with this the wing root areas must also be torsionally stiff enough to properly support the weight and aerodynamic forces generated between the tail booms and fairly heavy fuselage. The nose full of equipment must be heavy enough to counteract the weight of the tail booms and tail surfaces. This tries to torque the wing root area between the booms and center pod. So that area must be torsionally stiff enough to resist much in the way of flexing. In terms of a good solution for the booms I'd suggest that there is nothing at all wrong with carbon tubing. But for each boom do consider the use of TWO tubes separated vertically and joined to form an I beam like structure with a central web of some sort. Something of the sort will provide the sort of vertical stiffness you require while still being quite light and strong.