If you are going to add value to any of your products, then you need the Manufacturing license. If you are going to buy AR lower receceivers then assemble them into complete guns, you need a Manufacturing license. Likewise, if you are going to buy 1911 frames & slides & assemble them into custom guns, you need a manufacturing license.
You will have to remit excise tax for the guns manufactured. If you build less than 50 annually, then get a free ride on the excise tax. Over 50, you owe the tax. Either way, you still have to file a mfg. report. As a manufacturer, you will have to register with the State Department and pay the annual ITAR fee $1750.
You will need a letter from the city or your HOA stating that there is nothing prohibiting you from operating as a firearms manufactuer in your location. Even if they don't issue occupational licenses. (ETA, your home business license should suffice but BATF will call the city to verify that it means ANY business, if you can head that off in advance, do so so).
You will also need "posted hours", this is the time BATF can legally come unnanounced and perform a compliance check.