This one ticked all the boxes:80+ shots at 20 fpe, really quiet, light, front cocking, free floating BSA barrel, 10 shot magazine. Bought a used Gamo Dynamax for 200 eur, added some misc. parts + lots of sweat and curses and turned it in to a bullpup for around 300 eur total. I'm actually going to use that stock and rail system for another bullpup build shortly. What you could do is to buy a used Gamo Dynamax or BSA Scorpion and then buy a bullpup kit for that from guntuning. Total cost should be around 600-800$.
How did you get 80 shots out of it and how did it shoot?