You mentioned fat tire bikes. If that is one of your interests, make sure that your truing stand will handle fat wheels. If you can build a good fat wheel truing stand, that might be another market you could tap as not all truing stands will handle fat wheels. Another niche market might be wheels for recumbents, trikes, and adaptive cycles.

If there is a bicycle co-op in your area, a great way to get practice is to repair and rebuild wheels for them. They may even have a truing stand and other wheel tools you can try out before you make the investment. I started out truing wheels and replacing the occasional spoke at a bike co-op and now build all of my own wheels. Co-ops are also a great place to learn about bikes in general. Also check with your local bike shops, some sponsor clinics on bike maintenance and repair for the general public.

Being a machinist who is good at tedious tasks will definitely work in you favor building wheels. In all honesty, it is going to be hard to come out ahead of some of the mass produced wheels currently on the market. I build wheels because I have very specific ideas of what I want and I like doing it. Often the cost is about the same as if I had purchased a similar pre-built wheel and, if I was to sell them, my profit margin wouldn't keep beer in the shop fridge, much less give me a respectable per hour earning. I'd make more pushing a broom for minimum wage in the back of a bike shop.

If you are looking at a self-supporting hobby, maybe. If you are looking at a reliable source of income, it's going to be difficult (I didn't say impossible) to do starting on a shoe-string budget with retail components. Those independent wheel shops you see like Sugar Wheel Works Sugar Wheel on Vimeo don't buy their components one at a time at retail, they stock many supplies in bulk and have dealership wholesale agreements with component manufacturers. As a disabled veteran, are you able to tap into any small business entrepreneurship programs?

Good luck in whatever you decide to do