Almost forgot, 75-100watts per pound is the general minimum target to shoot for per pound as a starting point. On a glider, you should have very high lift and really shouldn't need much speed or thrust. It is a sail plane after all. If you want to get it high quickly and then thermal it out, nothing wrong with having good verticle performance to accomplish that.
I like to way over power my planes though. I am sitting abit over 125 watts per pound on my trainer and nearly 200watts per pound on my slow stick. My slow stick weighs 19.5 ounces as is and has 240 watts or so.