With a 3-in-1 tool, you end up with something that does everything but does nothing very well. Unless all you got is a tiny work space that won't take 2 bench tools, keep the mill & lathe separate. Yes, sooner or later you will want a mill too... and a metal bandsaw... and a tool grinder... and a boat load of lathe/mill tooling. What you pay for the lathe machine itself right now is just a small part of what you'd end up spending (t's just like RC airplanes, isn't it?) - so keep that in mind when you compare prices and think about how much to spend.

I wouldn't recommend a cross slide on a drill press for milling. It'll be like suffering with a jigsaw on jobs that're better suited for a tablesaw. Get the right tool for the right job.

By and large, any of these 7x 8x and 9x will do most everything RC engine wise you'd want to machine. Going larger, though, is never a bad idea.