glazing

I'm a retired carpenter/builder and I'm building a stickbuilt greenhouse and planning to use some 1/4" plate glass and some 1/8" (I think) tempered glass that was in sliding door storm panels. First should I use rubber glazing blocks at the bottom of the panels? Second, I have some second hand dual wall poly carbonate panels and I was thinking of putting that on the outside of the glass, with a small space in between, for a bit more insulation and protection from stones thrown by the lawnmower. Will I run into condensation problems even if I leave some spaces between the panels at the top and bottom? Will have those two layers reduce the UV gain? All of the glass is 40 or 50 years old so i doubt that any of it is Low E. Thanks for your attention, Larry