I actually just picked up a small hot wire cutter from Micheals. It cost 19.99 and it works well. It is an electric version and plugs into the wall, it has an on/off switch.
It heats up VERY quickly and cools just as fast. The only thing I would worry about is the Type and Denisty of the foam that you are going to use.
I have used it successfully on reg packing styrofoam and some of the higher density foam. (the high desity foam is a lot slower to cut, you pretty much let the wire melt through at it's own pace.) While the regular foam is more like a hot knife in butter.
Flower foam and dry foam are a toss up some will work with the hotwire cutter and others simply will not. I have bought large foam blocks online from different places and some work and some don't it is a mystery too me. They are both versions of the dry flower foam....so be careful when you order foam. Check with the website if it can be cut by hotwire or if they recommend a regualr saw. (Which is Hell)
I can also recommend this stuff Foam Putty
It is a AMAZING to work with. Covers and smooths out everything....and you can sand and paint on it as well. I wish it came in a larger tub though. I use it on all of my custom bases and I sculpt a lot with it on a regular basis.
Michaels also sells a foam specialized paint series in basic colors that also can be used to smooth the surface before you airbrush (this is in the foam section next to the wire cutters). But nothing compares to the miracle of the Foam Putty.
Here are some sites that I look at one is Styrofoam Tips
and the other is a foam prop studio they answer e-mails with questions or so I have been told. FoamProps