You should be able to swap the free float barrel nut for a delta ring with no issues. As you have pointed out there is lots more to an L119 A1 than a stock (it's not an XM stock in fact) and hand guard .
I would say if you are planning to go the L119 build direction then I would say plan to do a complete build , start with a base gun that has a delta ring, hand guard and triangle sight, then get a list together of the parts you'll need and the details that are very different to a generic M4. That way you wont be buying parts twice ! There are a few of us on here who do L119 builds who can help with the details and a shopping list.
Good luck with the build.