Old watchmaker lathes, or very small lathes like the Proxxon FD150/e look affordable used, and not very space consuming either.
Can these be used, by a non expert, to build aluminium parts like thread adapters between 20 and 80 mm in diameter, threaded retainers, spacers, tubes, from aluminium or brass bar stock?
The Proxxon doesn't look like it can cut threads so that won't help you in that area. As far as the other items you want to make it looks good. With practice you would probably get good results. I learned how to use a lathe in high school shop class and it's not hard. Some of the other much more expensive lathes can cut threads but with optional equipment.
