I would get a soldering iron from somewhere cheep, if your in the UK then Argos do cheap ones, if not then try HomeDepot, Its a great tool to burn in scratches and battle damage or just to add fine details.
Get a craft knife, i recommend ones that look like scalpels, you can be more accurate with them when your cutting and then blades tend to cut better than box cutters.
Acrylic paints to add details but I always spray and matte black coat on the foam before i start. It seals the foam a bit better and helps keep the rest of the paint layers on. Get some old sponges to help blend the paint if your looking to make it look dirty and worn. Some silver paint that you can use the dry brush technique to add scratches and the like if your looking for a more wore torn sort of look.
Other than that the only real expense is getting a Dremmel. They aren’t nessessary but if you want to sand/smooth certain edges it will save you a hell of a lot of time, they are so useful for costume building. I think thats everything really.
Oh before i forget, you can use superglue to bind certain parts but i found from early tests that it can make the foam brittle so i suggest using a hot glue gun, it basically melts the foam together and creates a great connection that is still flexible but you know its not going to fall apart when you move around.
I hope that helps you out. Any more questions then please ask away.