Very cool game in many aspects!
A few suggestions:
'Interact to start' sign is too high. It feels a bit odd to be next to it and enter menus... Could it be a portal on the ground maybe? I also bothers me that I have to be facing the wall when action starts.
Make more expensive weapons better. It was bit of a downside to buy a new weapon and realise it wasn't good enoug... Also would be nice to buy more hp / health packs. :)
All in all very solid experience. Well done!