I'd say the shortblock/ rotating assembly. it sets the tone for the rest of the build and if you have a motor on the stand it'll motivate you to keep the chains moving.

Heads are generally easier to procure "on demand" and sad to say with the economy the way it is you might find a bargain on some really good stuff the longer you wait. Of course you can buy a rotating assembly already done but to be honest I'd find the right block and gfet it machined with the first ~1500-2000, that way you know what you got and where your going. Guys buy heads and they sit inthe closet or worse the garage where anything can happen to them in 1-2 yrs.