Dakota Tactical has received very good reviews and has a loyal following, however, if you can find an HK94 that has been converted to the specifications that you wish, buying a German gun is never a mistake if it is with in your budget.
A German HK94 with a shortened and tri-lug barrel, paddle mag release, full auto bolt carrier and restamped (before it was a Tittle II firearm ;-) will most likely set you back a littler over five grand. Which added to the thirteen grand for the sear, starts to be real money ;-).
If you are looking for both the MP5 and the sear (or a trigger pack ;-) then you might find a combination.
Good Luck and welcome to the board.