First thing I'd do is establish a Trust, even if your local CLEO is friendly to NFA toys. This allows you to avoid fingerprints, sign-offs and makes your transfers go quicker (to make an SBS/SBR all you need is Form 1s, Citizenship paperwork & copies of your trust. For a MG transfer it's the same things, with just Form 4s).
If your buying the weapon in state from an individual they can Form 4 (means a regular indivdual owns it) it directly to you (no dealer involved, but dealers will try to tell you otherwise).
If your buying it out of state, you need a Class III dealer. When you find a weapon, first find out if it's on a Form 3 or Form 4.
-If it's on a Form 3 (which means a dealer has it) than it can transfer directly to your local dealer in about 3 weeks (but if you have the serial # you can send in the Form 4s for your part to speed it up).
-If it's on a Form 4 the current owner will have to transfer it to a dealer in his state and someone will have to pay $200 (probably you, but that depends on the individual) to transfer it to the dealer. Once it transfers to their dealer, the dealer can transfer it to your dealer, and than he transfers it to you on a Form 4 (and you pay a $200 tax). So in the end there will be a total of $400 in taxes payed (everytime it moves on or off a Form 4 there is a $200 tax to be payed).
I'd recommend getting one instate so it's simpler and goes ALOT quicker if the gun is on a Form 4 (getting mine out of state took me 4 months)
Typically you'll pay the full balance before the current owner files any paperwork to move the gun in case you flake out. If your buying an MG from a stranger 1/2 way across the country you certainly will want to take precautions against getting ripped off, but most people who have bothered to enter the NFA game are good people.
Once it transfers to your local dealer they will hold it until it is approved and transfered to you, while you constantly obsess and call NFA about it's status and stop by the store to play with it and fantasize some more, you buy accessories for it, order suppressors, etc, etc, etc, etc........
The apporved Form 4 will be sent to your dealer, and once it arrives he'll let you know, and you'll fly down the road to get there and pickup your toy. You'll fill out a 4473, but no NCIS check and you'll walk out the door with a MG.
Buying an MG was the best gun-related thing I've ever done and it's not like their going to get any cheaper. GO FOR IT! (your enlarged penis will thank you)