I would choose a 22 caliber semi automatic rifle like the ruger 10/22 because it's reliable and the ammunition is plentiful.
For a sidearm I would use another 22 pistol, I would use a ruger mkII because it's equally reliable and it has amazing accuracy.
For a "Melee" weapon I would carry something useful like a crowbar.
I chose these because I can actually purchase them, and I already own weapons like them, but mine are "Knock offs" of sorts, jamming a lot and what not.
All together it would only cost me around 300$ if I got them used.
Ammunition I could get around 5,000 rounds for 100$