Gentlemen -Its worth saying that as far as engraving goes you have mp5 and mp5-n. So it really depends on which you want. Most will make the decision of those based on if you have a 3 lug barrel or a 3 lug barrel with threading at the end.
Who are you going to have do the work? Also personally while it might take longer I'd have a smith do the conversion and form 2 the gun. That way their name is on it, not yours as they are collector guns that will hold value.