Yes, you would need to get a PDW barrel. Real ones are practically what is called "unobtainium" and if found usually more than $3k.
However, a B&T PDW can be had from HKParts.net for something between 400 and 500 the last time I looked and RCM for a couple hundred
less I think. I don't think that you would need a new trunion although they can wear out but one would have to shoot an awful lot of rounds
before that happens, I think. More than that though you would need the patience of Jobe. Sent an HK 94 out in October to have the barrel
pulled and some engraving done. I will get it back on March 7th -- a good 5 months.
Good Luck on your project.