LMT is considered a good company. Near the top if not the top of the line. Any makers standard parts (trigger group, bolt, carrier ect ) should drop in and fit with out grinding, or fitment. I built my Stag lower in about 20 minutes really taking my time. Its so simple most anyone can do so.
installing a barrel and extension takes some special tools( something I have not done) but is doable too.
Check out AR15.COM for step by step pics and instructions on how to build as well as lots of advice on parts, ect.