Welcome to the site. The only reason I could see for buying an all in one would space limitations. I have seen videos and forum threads where they are, as said above, okay but not great so if you have the room and power for standalone machines that would be the way to go. Some of the members here have stand alone machines and are working out rooms the size of closets so it can be done.
You didn't say your location but there is a Craftex lathe and mill on CL Nanaimo $7500, seems a bit hi to me look in good condition and I saw another set somewhere for $6500 but I can't find it now.
If you're looking at $8k then stand alones are the way to go. If you have your heart set on a 3n1 then there are used ones around for a lot less $ if you're okay with used.