Can one buy a MP5SD in the US?
Legally, yes. For roughly 30-40 thousand dollars these days.
There's a few companies making receivers now that are ready to accept a pack.
http://www.atlanticfirearms.com/component/virtuemart/shipping-pistols/om9-d-br-with-rail-detail.html?Itemid=0
Of course. They are quite common:
http://dealernfa.com/shop/mp5sd-a3-sh-sear-suppressed-dyer-excellent-1151-11891s-11891/