Can we buy every gun, upgrade them and still get this trophy? It's unclear if the game permits us to own every gun or if it checks if we actually used them on enemies.

i think its only if you USE it on enemies, so i believe that you can still buy and upgrade any other weapon. 

"Can we buy every gun, upgrade them and still get this trophy? It's unclear if the game permits us to own every gun or if it checks if we actually used them on enemies."

Thats one to get by playing on easy I think.

"Thats one to get by playing on easy I think."

Its definitely doable even on hard....I mean , especially with the Plasma cutter only there are some advantages. You'll get a mass amount of ammo drops from almost every enemy for using only that weapon, and upgrading just one will make it all that much better, once you get used to how fast you'll die on hard(even at around 50% health you can be torn in half instantly) then you'll be ok.