What Greg said is true but I want to hit a different angle as reading over your post, I think you are only going to need 2 stamps.
So first, suppressor:
-Get the stamp, you are good. You can switch the suppressor to whatever firearm you like when you like. Once you get the stamp, you are good to go.
-Since the barrel is shorter than 16" overall length you either have to run a pistol configuration or submit a form 1 and $200 tax stamp fee. Once you get approved, then you can purchase a buffer tube capable of mounting a stock. Until approval, do not buy a buffer tube for a stock much less a stock if you already have a buffer tube. Having SBR capable parts (Buffer tube and stock) in your possession while pending a Form 1 is still considered intent if they are not already on another rifle.
-With the Sig brace you have a pistol so no stamp needed.
So in total, 1 form 1 for the SBR and the form 4 transfer for the suppressor so $400 in stamps.