Before you do ANYTHING, get the rifle in hand and shoot it first. Then you can decide if it is as it was promised to you (barely used with ~30 rounds down the pipe) and will measure up to Armalite's accuracy guarantee of 1MOA with match ammo (that is one of the BIGGEST under-guarantees in the industry as I have yet to find one that wasn't easily sub-MOA capable, including some less under 3/4MOA, with proper ammo/shooter). If it meets your expectations, then you can consider getting the barrel properly threaded for your can, as well as any additional updates you might want/need at that time. Out of curiosity, is it by chance one of the Lothar Walther (LW) barrel models? Those are getting few and far between and are quite nice even by Armalite standards!
I wouldn't say that you HAVE TO HAVE the SASS conversion for shooting suppressed, but at a minimum, you'll likely want an adjustable gas block to tune things up for both suppressed and unsuppressed fire. Plenty of quality blocks like those from SLR Rifleworks as one great example that will serve you extremely well for ~$100, plus the cost of a straight gas tube at ~$20. You can go from completely off to wide open to ~15 click adjustments in between those ends with the SLR and in my experience, they are truly excellent products. You'll need the DA8 Clamp model for your 10T...see here:
DA8-C Clamp On Adjustable Gas Block
Where you go from there in terms of optics, buttstock swaps, grips, etc., etc. is purely a matter of personal preference.
Good luck and congrats on the new Armalite!