Arrow keys and space bar mappings are fine, but I would change your fire button to something like CTRL. Enter should be reserved for confirming choices and navigating menus.
As for the equipment groups, it's a bit hard to imagine without a screenshot, but the number keys would probably be your best choice. Most PC gamers are going to associate number keys with inventory slots.
Also keep in mind that if you are planning on developing your game for release in foreign countries, the keyboards may be laid out differently (usually the letters).