In my earlier post I tried, without too much success on how to negotiate for your equipment. If the tooling is there, try to get it thrown into the deal, if no tooling is available, you really need to consider the price. Don't be afraid to walk away; do it with a smile so that you can come back if necessary.
It took me 6 or 7 months to find my lathe, only went to look at one other. The mill took years, but I was willing to wait for the one that I wanted at MY price.
BTW if you come across a Wells-Index or Index mill, they are really good mills (I like them better than Bridgeports). Still in business and if the quill has an old style taper they will regrind it to a more modern one for only a few hundred dollars.
Back to your question on accessories. Lathes have a half dozen or so standard spindle mounting styles. Your South Bend's and Asian clones will have readily available backplates/chucks.
If you are in the market for a chuck they can be purchased with or without the backplate that you need.
The tool posts are sized for the size of the lathe, so don’t be too concerned if you need to purchase a QCTP (quick change tool post) other than spending the $$’s that is.
The mill is a little different, many mills have a R-8 spindle taper and your tooling will need to be R-8, most bridgeports and clones as will as HF mini mills take R-8 tooling. Older and or bigger industrial machines may take nmtb 30, nmtb 40, nmtb 50 spindle tooling (nmtb=national machine tool builders). You should avoid a mill if the spindle has a different taper than one of these, unless it is an Index and you want to have it re-bored.
I hope that this answered some of your questions,
Mike