you know, a QCTP would be awesome, as would some indexable cutters, but as you already have the holders and a bunch of HSS bits you could easily make do with those for a while. Nothing wrong with them and changing them out might take more time/ fiddle than a QCTP, but if you're not in a production environment it's not a big deal.
First thing I'd get is a digital readout system in place. Makes life a lot easier and you can get a triple set up (X, Y and tail stock) for $100-150 from DavidH on here or plum a few benjamins more for a system with a single readout. I'm not an expert on that though, so ask around. Adding a digital read out to my lathe (orders of magnitude smaller than yours) has made it a much easier to use tool.
Do you have a bench grinder/ diamond hone/ arkansas stones already? If not, those will be super useful for grinding or touching up your own HSS bits.
A cut off tool is very useful, you could grind one of those yourself if you have a big enough spare bit/ blank.
A live center would be nice, as would a steady rest if you're going to do stuff longer than 10in or so.
Most important of all, a bunch of different types of material (scrap yard? Machine shop castoffs?) to practice on! That's the best way to learn
