Something with enough ooomph to light a full-length portrait in a smallish room.
What formats are you using? you could easily get away with a 250w/s head if you're using medium/small format. I use a set of 500w/s heads which have plenty of power to allow you to stop down using 4x5... it's all relative... and what sort of price are you looking at? We might have some of our older 250 heads coming up for sale at the studio soon...
My goal in life, is to be as good a person as my dog already thinks I am.