What material will you be shaping? PolyU or EPS? There are some difference in tools. The basic arsenal will include a planer, a surfoam, a block plane, a spokeshaven, a level, a square, the large calipers (for measuring board thickness, pencils and a MILLION sheets of sandpaper/dragon skin
I would also buy a good quality respirator - the small foam particles can get into your lungs, and when you work with some of the chemicals during glassing you'll need it later.
www.Foamez.com has a pretty good selection of tools and I think they have a starter kit.
http://www.foamez.com/index.php?main_page=index&cPath=2_59 You may have most of the basic tools in your shop, already. Things you don't have you can make. The calipers as an example, can be made from some pieces of light ply.
The one tool that you will use repeatedly is the electric plane. Spend some dollars on this and make a good investment, it'll save you countless hours and frustration later. You'll want a carbide barrel for EPS so you don't tear the stuff.
That'll get you started and welcome to the obsession!
Keep us posted on your progess, please.