Hi there, welcome aboard!
If you want to have the barrel threaded, you will need to find a good reputable gunsmith in your area that can do it, as far as cost goes, well, I have heard just the threading job is going from $50-$150.

ALSO, the alternative is the barrel adapter that you mentioned.
Click on this link, and scroll to the bottom of the page, and you will see it. Another guy here has one on his, and I think it looks pretty good. Just scroll down a little and you will see it.
M&P 15-22 Picture Thread
As far as what suppressor to use, I am not sure myself, but I did see this one, pretty in-expensive. Don't for get, you need a crush washer also, I think that's what it is called.
Hope this gets you in the right direction!
Gunsmithing - Shop Gunsmithing Tools & Supplies at MidwayUSA
M&P15-22 Barrel Tools and Accessories - M&P15-22 Parts, Tools, and Accessories