Couple of thoughts:
There are lots of places that sell pre-converted asian mills and lathes. You are not restricted to Sherline for off-the-shelf small CNC machines.
If you are starting with hex stock, you should consider getting a lathe large enough to pass the stock thru the head, otherwise you are committing yourself to pre cutting each piece and the resulting waste in stainless material and manhours may cost more that the lathe itself.
Of you are willing to precut your stock and don't care if it can pass thru the spindle, you can consider doing the threading on your mill.
For many years I did extensive machining of all of the stainless grades including Inconel and Monel. I would wager that you will have zero success cutting anything except perhaps 303 on a Sherline in a production environment. Stainless work hardens pretty much instantly so you need enough power to keep cutting at ALL times. For this you will need not only the HP, but a very rigid machine, even for 12mm shafting. You are also goint to need flood cooling.
Single pointing 316 is one of the trickiest things I ever had to do. Unless there was an absolute need for single point (reverse thread, odd size or pitch, thread to a shoulder, etc ). I always used threading dies . . up to about 25mm, using a die holder in the tail stock. The low speed also eliminated the need for flood coolant. A squirt of threading oil was sufficient.
You make some beautiful parts, incidentally. How are you cutting the hex pocket in the drain bolts?