Norman,
How fast will it go? I bet it won't go fast enough!
Whether you use high speed steel or carbide depends on the practical speed limit of your machine spindle. Carbides will stay sharp longer, of course.
I think the most important factor about the tool geometry is to select a "high-helix" endmill. Lots of guys rave about 3 flutes, but 2 flute may be adequate if you have limited feedrate capability on your machine. Lots of flutes is better for high feedrate finishing, maybe not so good for roughing (they may plug up...it depends on how good you can get the coolant into the tool.)