Diana made some really nice air rifles in the 60s and 70s. They were all similar in lineage starting with the model 35. This model upped the power of the model 27 and was one of the first "magnum" springers capable of 700 fps in .177 cal. The model 38, 40, 45 followed and are basically the same power plants with slightly different bore/stroke. Power increases with model/age. I believe there is a very nice model 40 right now on Auction Arms or Gunbroker - just saw it the other day. You should be able to get something like that for around $150, give or take. These are sweet shooting guns, the ones up until the early 80's had leather seals. With proper care, they should last a very, very long time. Most have very nice triggers and are lacking the plastic parts of modern guns. There should be no problems with these guns holding scopes and the stock open sights are quite servicable. Here is a 1978 model 35 I picked up for right around a C-note. Easy cocking, sweet shooting and solid. What more do you need?

"but I'll be needin' that gun, fer squirrels and such."