If you're already talking to a shop that will cut/thread your barrel and build you a can, just tell them that you want the can permanently attached to make the barrel length 16" and they should easily be able to do it for you. I personally wouldn't go with the one stamp gun because once you have a registered lower you can put any short upper you want on it (regardless of barrel length or caliber) but I know of people have permanently attached cans who are happy with them.
If you are doing a SBR, I would definitely go with a forged lower. There are stories of poly lowers breaking under use and whether or not they're true, there's no reason to risk it when you're registering a NFA weapon. If your registered SBR lower breaks, you're out of luck and have to register another lower (including another $200 tax and wait).
For transferring the suppressor, assuming your local shop is a manufacturer with a SOT (allowing them to make NFA weapons) they will make and register the can, and then once it is made it will transfer to you on a form 4. They should provide you with the form when it is ready and then you add your information (trust info or individual info) and a check, and then mail it to the NFA branch. If your dealer is not a manufacturer and just a dealer of NFA weapons, the process will be almost exactly the same unless they have to order the suppressor, which means the can will transfer to them on a form 3, and then once it arrives they will be able to transfer it to you on the form 4.
With the SBR, you can do it two ways. You can give your shop the gun and (assuming they are a manufacturer again) they can register the gun as a SBR and then transfer it to you on a form 4, just like the suppressor. The second way, which seems to be most common with ARs, is to register the weapon yourself on a form 1, and then add the new short upper or barrel yourself. Note you will have to wait for the approved form 1 and tax stamp to come back from the ATF before you make the changes. The advantage of doing a form 1 is that you get to keep your rifle in a functioning (non SBR) condition until the registration paperwork comes back.
Now for some not so great news. Form 1s and form 4s are now expected to take around 9 months, and form 3s (dealer to dealer) are around 90 days. Even with the wait, I would still recommend you go ahead with the project if you really want a suppressed SBR.
