Welcome to the forum!
How extensive are you wanting the inventory system? I know it is possible using just logic if you do not want RPG level of inventory (That is still probably possible, but it would be a huge spider web of logic bricks). The only problem with logic bricks is usually when I make something like that it is fairly unstable if someone is button mashing (like in my FPS game if you switch to the sniper rifle too quickly it can cause it to break).
If you are looking for something similar to Jedi Academy where you get items and click 1-9 to switch between those items I might be able to throw a prototype together with logic bricks (though you may want to wait for someone with python knowledge to come help you out)
I am super excited to see your game coming to life! Good luck and have lots of fun
p.s. I just remembered that I had a small example of this I was working on for a game (This game has left development for now, but If you want to take a look at the logic feel free )
edit: I just realized you said UPBGE- this game is in BGE 2.79 but the principles should move over to UPBGE too.
The controls are WASD, Space, Mouselook, Left mouse button shoot, right mouse button scope (on sniper rifle) 1 for machine gun, 2 for sniper rifle- this game is very much not ready for players… but it works as long as you don’t button mash a ton