As far as rifle suppressors go, in my somewhat limited experience (I have 2 rifle suppressors), I'd look at some reviews and see what is well rated.
Suppressor info can be a bit tricky though since a lot of people don't have much experience with many suppressors due to all that is involved, so you might get a glowing review about a suppressor, but it's from a guy who just got it as his first, and is super excited about it, but hasn't shot others to know if his is actually good in comparison.
Anyway, with that out of the way, I'd recommend going LIGHT, and not overly long. Made those mistakes with my first, and quickly bought another for a more hunting specific type can.
Buy once, cry once is applicable here.
As far as purchase- my first one was through a local shop with a kiosk to do fingerprints and picture. They walk you through the e-file.
Second rifle can I got through Capitol Armory online. I mailed in fingerprints to them that I did myself at home. You upload a photo, and do the rest online. You connect with them on a computer and they walk you through all the online paperwork. Overall maybe took a little longer than the local shop but very doable, and the suppressor shipped directly to my house which is also convenient in some ways.
I believe you need to have the shop complete the e-file either way, so plan on that. (Unless you do a Form 1 and manufacture the suppressor yourself, in that case you can complete the online filing yourself).