Buy a stripped upper from Brownells, they have the best price, get the one WITH the piston bushing installed. San Antonio Laser will engrave T-marks and any other marking you want, if you even care about those.
A Brownells barrel will work fine unless you absolutely need to have a chrome lined bore, then Marvin Pitts over at Nefarious Arms is the only place you can get them outside of dropping the dough for a legit 416 barrel.
Brownells, HKHaus, and HKparts
will be your friends, in that order, if you want to do this. Also no shit go on HK's webstore, they sell quite a few parts you'll need and they'll happily help you if they can. Gatewood Supply may have a few things you need too.
I'd be happy to help if you have any other questions.