Howdy
The best "out-of-the-box" spring is probably the wolff 24lb. These seem to work good for the original Detonics guns, I have never used one in the NewDet guns. I think Wolff lists them on the web-sight. I replace them about every 700 rnds. These short cycle guns are HARD on springs. As far collector value, it really depends on who you are dealing with. If I could get one of these guns cheap enough I would probably buy it just to experience it. I would probably end up selling it again. I like to shoot my collectable guns and have lost money doing so but what good are they if you don't bang them once in awhile!
Hope this helps!
L8r
Conley