The G&P kit is probably the best way to go. The shrouds are quite thick and easily skirmishable. Probably more so than the vac formed ones that you can get.
Upgrading it is another matter,
On mine I have made the following upgrades
From Phil S I got
Metal vents
Metal Triangle support
Metal Front block
Metal Stock
From the classified section on this board I got a Real Spas cage and grip. You need to be here a little while and make a few posts to get access to that part of the board.
I bought a Maruzen M870 gas shotgun.
Either the Cyma or a Marui Thompson will do fine.
There is a lot of modding required to get all these parts to go together however.
The maruzen shot gun needs to be completely stripped and modded. The spas cage needs to be cut in the correct places as well as shortening the original grip.
To connect the launcher section to the thompson there are two ways I am aware of. First is the adapter that member Tommin used to do (M41.com) or there is the custom block that member DATA does. The latter also incorporates the light gauge counter from the G&P kit. You have to remember that with that kit as soon as you replace the front triangle there will be no where for the light gauge to go ( this is what counts the pellets and makes the counter go down), Data's block nicely gets around this problem but does required further mods to be made to the inner barrel of the thompson.
My barrel is the original G&P one but turned down in order to fit the Phil S front triangle.
The stock, if upgraded, will need modifying in order to either work with the existing G&P locking mechanism or a new mechanism (which DATA also does) will be needed. The stock lock mechanism on the G&P kit is it's weakest point really.
For the shotgun also you will need a custom gas tank, unless you go with the Tanaka version of course. DATA also does these. (he's very industrious)
There's probably more that I cant think of right now but to cut a long story short, it is not an easy or quick undertaking to upgrade to a fully functioning airsoft Pulse Rifle but well worth it if you do

Cheers
Darren