shimano road and MTB cassettes are fully interchangable, except for 10sp only road hubs.
you can even keep the XT rear dérailleur since it'll work with the shifters.
I swapped my 9 spd mtb cassette to a road cassette, and kept my XT derailer, no issues.
I also picked up a wheelset, similar to what the OP priced. I got XT disc hubs, and Mavic 319, 36 hole rims, for $230 including shipping on Ebay (about $180 Euro). But my rims are wider, 700x28 min. I had to look a long time to find the set I wanted at that price, especially the 36 hole rims. Bottom line, I think you could get a better price on Ebay, if you can find that set-up.
I am also interested in the bullhorn/aero set-up. Anyone know if the bullhorn reverse brake levers are compatible with mtb disc brakes(Avid BB7)?