It would be a losing proposition unless you had a lot of people looking for a gunsmith in your area.
Right off the bat, you need to check your local and state laws and meet those requirements. Depending on where you live, it might not be anything, but when you get to the federal level, you will have to get a manufacturers license from the batf. If you have no real business, it's difficult to justify the cost. Still, it shouldn't be a deal ender if you really want to do this.
Next, you will need to invest in a mill and lathe. You don't have to have a cnc machine, but a dro setup would be recommended. The lathe should be large enough to accept turning a barrel on centers, and the tooling required to operate the machine WILL easily exceed the costs of the lathe. Still, you might be able to get a decent lathe/mill and basic tooling for under $10k if you search around. Just try to avoid the chinese junk. Look for R-8 tooling on the mill, and cooling/lubricating systems are something you really want.
Next, you will need an assortment of hand tools. Cheap screwdrivers that do not fit well will surely mar the screwhead itself and possibly the firearm. People don't like that. Also, pipe-wrenches should never be considered as gun tools
You will need a space to set up the workshop then. Since the lathe and mill are pretty heavy and require a solid foundation, a concrete floor is essential. You will also need a safe to store the guns in, so figure that into the equation.
The majority of the work you are likely to see will include re-blueing, crowning barrels, drilling and mounting scopes, barrel swaps (need headspaced), and the inevitable "my gun is inaccurate, fix it", and the "learn how to shoot" response is not an answer.
Try going to this site and nose around. It might help you decide if you want to pursue this Gunsmithing