For motorglider soaring you are more concerned with sink rate than L/D, because the engine is thought of as an L/D substitute and you are already penalized with a higher sink rate than you normally would because of the weight of the engine and fuel.
The L/D, and the speed at which it's achieved, will determine your ability to get between thermals without having to start the engine, but basic thermalling climb performance is a function of min sink rate and the thermalling speed. (If you've flown a Schweizer 1-26 which has an L:D of only around 23 but floats like a butterfly and can stay up in very weak lift because it can slow to the low 30s, you know what I mean).
On a good soaring day you will get thermals of up to 600 or 700 fpm and on boomer days over 1000 fpm, so on such a day a glider with a 300+ fpm sink rate can get by quite well. But as the conditions get weaker late in the day, or on a marginal day, you want to be sinking not too much more than 200 fpm. If the L:D is not so great, it just means you will be starting the engine more often to get yourself out of trouble trying to run through sink between thermals.
As far as Ultralight gliders go:
The Sandlin Goat soars fine with an L:D of somewhere around 10 (there is no published figure - also, the basic design does not accommodate an engine so you'd have to engineer it). It's sink rate is well under 200 fpm and can thermal in weak lift and Goats have done pretty long cross country flights if the winds aren't too strong.
The Mitchell Wing had an L:D of 16 with engine and a min sink (I dimly recall of somewhere between 200-250 fpm) good enough for soaring in most conditions except very weak days or late afternoon.
The Marske Monarch was probably one of the best performing ultralight gliders that an average person could afford and could be motorized with an L:D of around 20 and a min sink well under 200 fpm even with an engine.
Then there's the guy who put two Hobby King RC electric motors on a Millenium, which is around 18:1 and I believe sinks under 200 fpm.
There are ultralight gliders with much higher L:Ds but they are super expensive factory builts, way out of my league.
For a homebuilt ultralight motorglider, I'd be happy with an L/D of better than 15, but preferably better than 20, and a min sink of under 220 fpm. I'd power it with electric motors. Instead of depending on L/D and penetration power, you have the motors.
You could scratch build a Mitchell Wing and power it electrically using RC components for probably under 10K (if you can find plans).