Alright so I've decided to do a playthrough using just weapons and gadgets to get the achievement for killing people with every type of weapon and gadget, while also getting the achievement for not upgrading any of my abilities. Now I heard you can't get or upgrade everything, but I'm assuming there must be enough for every type of gadget and weapon? How should I go about this?

There should definitely be enough money to upgrade your gear. Just explore and try to loot every guard, Sokolov paintings are always worth good money, and keep an eye out for safes.