Good call on the midlength gas system. I think you'll like it.
The Mk18 isn't currently being sold as a whole right now. DD suspended commercial sales indefinitely while they catch up with SOCOM orders. They sell the individual rail, barrel and low-pro gas block separately, though. However, you would need to go through the NFA process for the rifle since it's a 10.3" barrel. (My current build is essentially a Mk18 variant, but I went to an 11.5" barrel for practicality reasons.)