JonJon -- I'm far from being a scope expert, but I recently went through the same decision for the same rifle. My conclusion is that you probably don't need to spring for a very expensive scope, as the main benefit over cheaper scopes is sturdiness, and recoil is not an issue here. I looked at scopes by Weaver, Bushnell, and Tasco, and finally got a Bushnell 4-12x40 which I was very happy with. I personally wouldn't go with a larger objective than 40, or you'll have to mount it on high rings -- the 40 mounted on the supplied Ruger rings (?medium height?). I did think that an adjustable objective is useful here, since most non-adjustable objectives are set at about 150 yds, which is a little optimistic for this rifle. This may be especially useful if you use it for target shooting at 50 yds. I would recommend going to a large store with a lot of scopes in stock, and look through a bunch of them; pick the one you like best, which has the clearest image, and it'll probably be sturdy enough for this application. You should probably be able to get an acceptable one for less than $150. YMMV, and others may have differing opinions. Best wishes.
Andrew