If you go with carpet, I recommend looking at a local carpet store for auto carpet, it's the same stuff you get at Bass Pro and such places, and compare the prices.
If you are going to carpet metal and areas that can't be stapled as you upholster, use contact cement, you can use it for all of the carpet for that matter, or use an outdoor adhesive (go easy on the amount, it don't take as much as the bucket label recommends for a thin carpet).
Also, avoid carpet with any loops in it (not much fun with fish hooks).
Just my $.02 I kinda know a little about carpet