I totally fail to see what international traffic in arms Colonel is proposing. He is not proposing buying or selling anything outside the country.
But I will add my usual note about not forgetting other federal laws as well as state/local laws and regulations, such as a business license, sales tax collection license, zoning law compliance, pollution laws (dumping the bluing tank into the creek is a no-no today), OSHA laws if you have employees, business tax filing, and on and on. Not to mention insurance, personal and shop to protect against both against loss of a valuable item while in your possession but also against goofing up and damaging a gun or making one unsafe.
I personally wouldn't try doing what Colonel proposes without incorporating a business for self-protection. A sole proprietorship leaves you too wide open on liability for my liking.
Now on the FFL. If you were to work retail, and simply modify customer's guns, you are probably not going to need a manufacturer's license. But if you buy guns, modify them and sell them, then BATFE says you are manufacturing and will need a manufactrer's license, not just an 01 dealer's license. If you work strictly for FFL holders, and do only limited modification, you MIGHT be able to work under your customer's FFL, but that is a gray area that needs to be discussed with BATFE.
Jim