I don't like the idea of “buying upgrades”, they did that in sega genesis, it was like the GB doesn't know how to built their stuff.
But the concept is good. I prefer the options to buy raw material. The firehouse will have a room where the player can built new technology. The player will need to choose the right material, first. The game will allow the player to create its own design or to choose one made by Egon and Ray. After choosing, a minigame follow. It will show the construction in progress (it can be anything: clip of gb working or your hands working on the tech), while the player follow the correct button squences. When the tech is ready, it will need to be tested in the “test room” (on a target, not a ghost).
If the product is ok, you will see a good view of its potential, whith a success message. But if the player didn't follow the buttons sequence correctly, or if the player didn't choose the right raw material, a lot of things may happen: nothing (a failure message appear), or alarm before an explosion (your character runs for cover, failure message). Anyway, everything may look cool but it will be on the battlefield that the final test will tell. Some ghosts may laugh at your new technology! The player may feel the need to carry one of its old ones on a teamate, just in case.
Sometimes, the best solution is not to make a whole new technology, but to upgrade the old one with a booster, for exemple. But the upgrade that you create may also waste the tech. You will lose it until Egon and Ray send you a replacement, if that happen.
So, if the player fail, money and raw materials are wasted. Some materials may not be available during a period of time too; if a danger is approching (like gozer was coming in gb1), the player better play safe or try to make money quickly. Why not add an option about ordering raw material from other countries, when it's not available around? The player will need to pay before receiving the stuff, was it a good choice to order instead of waiting that the markets gets their new inventory? Maybe the shiping will be late or its price very high compared to what came.
I like the idea of 3person and first… only if it's not a shooter based game.