Making your own parts can be a double edged sword. A really good mini lathe is going to cost several hundred dollars, but once you have it you now need to spend more money to buy the necessary tooling. Chucks alone can run nearly as much as the lathe sells for. You can make a simple knob similar to those used on view cameras by turning the brass in a drill with a file for a cutting tool. To make several exactly a like is next to impossible!
I believe the best way to buy any lathe is to find a hobbiest wanting to sell or trade up, then make an offer based on your own research to buy the lathe and all his existing tooling. You will need a 3 jaw chuck, and a 4 jaw chuck.
I like to use collets when working with small parts, so a collets and a good closer could be on your list. A grinder to sharpen and shape your cutting tools, and the list goes on and on. But you could get by with just these tools if your innovating.
I personally do not like to knurl a knob on a mini lathe because I feel the pressure necessary for the tool to cut the knurls deep enough is extremely hard on the bearings of the lathe. None of my mini lathes (3) have a back gear, so this contributes to my not liking to do knurling.
I certainly don't wish to discourage your project, but if you need several of anything CNC is the only way to go, but it is very expensive, perhaps you could sell the over run to others on ebay etc. I wish you well in what you are trying to do, but want to warn of a few pitfalls.