A fiberglass cutting wheel like
these are durable enough that you won't go through 6 of them cutting a foot long piece of thin sheet metal, but not as expensive as some of the metal or carbide ones.
Sanding drums (or bands as they call them) like
these are nice for cleaing up an edge you just cut. Just look at the store for a package of various sizes/shapes/grits.
These are the contoured grinding bits I was talking about... you probably won't have much use for them in case modding unless you plan on carving a design into a plastic case, but they do come in handy for trimming casting flash off the inside of a part to make it fit better.
These are an example of the die grinder style bits I was talking about.
And of course...
drill bits.