can i pick up these parts; body and cap
g2 body
cap

and then add the drop in kit
kit

im guessing all this will work or will i need anything else. looks like the best way to build one without buying a G2 then taking it apart

IYou can get a Surefire G2 LED preassembled cheaper than you can go the route you suggest. A complete G2 LED can be found on the internet for between $59 and $69 (MSRP is $69).  The G2 LED comes with the standard G2 body and tail cap with the metal head and Surefire P60L LED assembly preinstalled. I have a G2 LED on a LaRue QD offset light mount on my 6920. It was available as a combo from LaRue for $142.

There's no tailcap on that body, and the bezel doesn't say whether it comes with a lens or not.  Some G2s have plastic lenses - I have a few that are slightly melted from extended use with incandescents.  I can't see what's in the TNV kit, as their page won't load for me, so if it includes a tailcap, you'd likely have what you need.

You can get a complete G2 for $37 shipped on ebay.  Personally, I'd go this way, rather than saving a few dollars on parts.  Half-price, I'd consider.  You may want to check the marketplace over at http://www.candlepowerforums.com, as that's the place to get great deals on flashlight parts.