I used an Atlas/Craftsman 6x18 lathe for more than forty years.
While a lantern style tool holder is not as convenient to use a quick change tool post and not as rigid, the lantern style is more versatile. It would not be high on my list as a candidate for an upgrade. Regarding a milling attachment, IMO it isn't worth the bother. The work envelope is to small and you won't have the rigidity of a true milling machine. Go with HSS tooling to start and set up a decent grinder to grind your tool bits. The OEM lantern style tool holders use 1/4" tool bits. Even with a QCTP, I would still opt for 1/4" tool bits as they are less expensive and easier to grind and you won't really benifit from the greater rigidity of a larger tool bit on the 6x18. I often use 3/16" or 1/8" tool bits.
I bought some MT2 collets some while back and have never really used them. The bother isn't justified by any advantages.. Necessarily, the size of stock that can be worked is limited to less than 1/2". You also need to set up a drawbar to use them. For small work, a MT2 mounted three jaw chuck would be more convenient although you do need a drawbar to us one too. If runout on reworked parts is a concern, a four jaw chuck would be more useful.
In more than forty years, I have never had a chuck spin off when running in reverse. The lathe simply doesn't have enough torque to do so.