I really donno the stock what is like when compare the new and old version of TM M4A1... anyway I bought a second hand M4A1 and work from there... since I changed most of the stuffs already...
for the outer barrel... better to buy the one with full set... like GnP is a easy start... but again you don't have to buy it right the way...
and if you really want you could buy some nice outer barrel (around $1000) but unless you plan to only work on one airsoft... if not then work from GnP one...