I had experience with an RC balsa glider-type plane, for which I purchased the add-on motor and propeller assembly. I trusted that the attachment kept the C/G in the proper location just at the wing junction. I flew it several times until the ESC lost battery power. The wind carried my plane to parts unknown because I forgot to charge those little batteries.
Your glider will be slowest while you keep it lightweight. When you add the weight of a motor and prop, you must make it fly faster. Choice of motor and prop is forgiving because the load (moving air) is flexible. To slow down you can always reduce motor speed. On your terrain are you able to land at slow speeds? The extra weight can make you value repairability more than rigidity.
Try test glides after adding dead weight to your plane of equal weight to the motor and prop. See what airspeed it wants to go. See if it responds to control flap movements. If it doesn't then you need more speed.
R/C magazines and catalogs describe suitable motor choices for a plane's weight, size, wing loading, etc. A large prop spinning slowly is more satisfactory than a smaller prop spinning quickly.