In general, if you see yourself continuing with machining, you should invest in a mill. The size depends on the work you expect to do. If you strictly want to do miniatures and models or other small projects, then a mini mill could work. A milling attachment on a lathe is generally a VERY limited thing. Very small size parts, very small light cuts, huge setup time, etc.
Along the same lines, a lot of new metal workers get excited about a combination lathe/mill like a Smithy or similar. And some people do great work with them. Most people, however, find the mill portion to lack rigidity, are limited by the small 'table' and constrained by the work envelope. A lot of people who buy a combo machine soon upgrade to separate machines because of these limitations.
You said you have an Atlas 9 inch, right? This one?
Atlas 10-inch lathes from the first to last. Includes details of the British copies by Acorntools, Sphere and halifax
www.lathes.co.uk
I know Atlas made milling attachments for the later 6 and 10 inch lathes but I don't know about these models.
I think the 9 inch models were only made in the early 1930's? If so, you've got a really old, very light duty machine. Probably with a non-trivial amount of wear over the years. Asking it to do milling work is a lot. It seems unlikely that you're going to be happy with the result.
OTOH, you can use a 4-jaw chuck or face plate in creative ways to do milling-like operations with the lathe. For example, I mounted a rectangular part in my 4-jaw to create a t-nut (ie a round boss). So it largely depends on what you want to do.
Craig