I have had an ffl for many years so maybe I can help some.
Lots of advise here that is good.
You will have to comply with either zoning or deed restrictions
Get a sale tax permit
You would be liable if someone gets injured via your repair. If you have nothing, you could be ok, if you have assets, you should set up an LLC and possibility about insurance
Should claim income and expense on your taxes
Get theft insurance that would cover your customers guns. Your regualr homeoweners insurance will not!
You will have to log guns in and out and be aware ATF can and will inspect your records. Failing to keep good and accurate records can lead to ffl suspensions and worse.
Make sure you really know what your doing. I have seen folks who are handy thinking that can work on guns, just because they can work on something else. Usually after seeing what they really can do on guns, I can train blind monkeys to do better.
Best to you
John Boyd