Well, I went ahead and ordered the Mavic XM 719 Disc 32-hole rims with XT M756 hubs, DT Swiss db spokes and brass nipples from Universal. Hard to beat the price with the 15% coupon code (vip15). Comes to $310 including shipping.
Handbuilt and in stock, so they should arrive in the next couple days. With rotors, cassette, tubes and tires, I'll easily stay within my budget. Maybe will spring for a pair of Schwalbe Duremes.
I would have preferred a 36-hole wheelset and maybe a little wider rim, but that would mean a more expensive custom build. 32-hole should be plenty strong.