1: To answer your first question:
http://www.mp5.net/info/sbsconr.htm
according to this link, it is.
2)I don't have any experience with silencers, but as shaggy said, a suppressor is a fine tuned instrument, especially when you are dealing with a round moving at over 2500 fps. If you really want a suppressor, buy a real one from a reputable manufacturer, cause unless you are really handy (or an engineer), and have a cnc mill lying around, your suppressor will probably not be that good (five years from now you'll be in gun history for inventing the most efficient type of suppressor to date, knowing life). But seriously, building a suppressor isn't as easy as buying the parts and putting one together (since no one that i know of sells parts kits).
3)Not sure what you are asking? Do you mean register silencers after you make them, or pay the tax, and then buy your sliencer when your funds have recovered.
Again, i suggest saving up and buying a professionally made one. And yes, the professional guys can do all sorts of nifty stuff to your gun barrel to make it suppressed (integrally suppressed firearms are sweet, but you can't move the suppressor around)