How about fume extraction (laser cutter) and compressed air? Those would be best to plan for now. A compressor can be very loud and would need to be located such that the noise does not violate OSHA regulations.
I would not get a plasma. It is too dangerous in a crowded shop environment and you need a downdraft table with ample fume extraction, filtration, and ventilation.
I agree with the CNC router. I would choose this over a second CNC mill. What mill do you currently have? I would get some vises, a vacuum table, and waste board. Learn how to use the Work Coordinate System and layout multiple components/parts to do in one routine. A router that accommodates 4' x 4' sheet material would probably work out best as many local suppliers sell pre-cut material in this size.
The eBay CNC router designs can be hit or miss. The 6040 linked earlier does not look adequate for machining aluminum based on the gantry design (very small distance between the bearing blocks on the linear guide. Beware of other routers with a floating guide shaft (
example) as they are not rigid at all and the gantry can be deflected with the press of a hand.
Depending on the funds you have, a manual lathe or CNC lathe really makes no difference. Conversational programming a CNC lathe like the Tormach 15L Slant-PRO with Path Pilot will be faster, especially if using a turret tool changer or gang tool setup. What lathes were you considering?