To the original question - IMO of the 3 SMGs you name, the Swedish K has the best reputation and, in it's original configuration, would be the most desireable. I can't comment on the where or how of the legally approved location for the weapon's serial number.
There's really not much more paperwork for a Class 2 manufacturer than there is for a regular FFL or manufacturer - they simply have to mail or FAX in the new weapon's Form 2 registration within 24 hours of completion and maintain the appropriate paperwork for his ownership. He must also be a Title 01 FFL holder - a standard FFL. Other hoops to be jumped through involve the EPA, SSA, local zoning, etc.
A Class 2 SOT can make SMGs, suppressors, AOWs, etc. without paying a making fee. He can make as many as he wants for his R&D work. A Class 2 can also deal in NFA firearms. A Class 2 can own post samples, and import one sample NFA weapon for R&D use, with the appropriate police letter. He can transfer NFA weapons with other SOTs without paying a tax. I'm pretty sure that destructive devices (DDs) are not exempt, though.
The advantages are in the eye of the beholder, but if you want to make a MG-42 or MP-5 for your own use, it's certainly cheaper than buying a transferrable model. As with any SOT, you must prove a certain amount of legitimate business though - getting a SOT just to enhance your personal collection is frowned upon by the ATF and they'll take action.
