I could have been only half listening, but I thought remember hearing one of the guys on the bombcast talking about some sort of shop that has all the old stuff in it from all of the other games. Is this true? If so, where is this located because I have yet to see such a thing.

A shop? No... When the fuck did this happen? Between Drunk Grunt and Nerdgasmic Traynor, I don't remember any of this shop.!

I think it might be those terminals in Armax Arsenal where you can unlock party members etc? If I remember correctly you can purchase all weapon upgrades from there also.

In the arena place, there are two terminals near the weapon bench IIRC. One gives you access to the gun upgrade system and the other lets you purchase items from all the stores in the game. I'm not sure if this change goes back to the stores themselves, but when viewed through the terminal in the DLC these stores all have significantly more items to buy than they did previously, mostly in the form of additional levels of weapon mods.