Nice idea to setup your own machining center. Vertical Integration is the need of the hour. From your point i understand that you will need to machine from a min of 300 pcs to 5000 + pcs. The size or the brand of the machine greatly depends on how big your parts are and what kind of job profile to be carried out. There is no particular machine which can machine only aluminium .... you can machine MS , castings etc. but it all depends on the quality,productivity and the size of the machined component. I would now suggest you to do some small paper work before you proceed.
1. Chart out size of Minimum sized component size to the maximum size.
2. Operations needed to be carried out. example some compnents may require 5 axis machining.
3. budget which can arranged because you will need auxilary / support equipment for the machine shop to rationalise productvity and remaining cost effective.
4. Quality control / measuring instruments.
5. Man power needed
If you can give me some drawings of your smallest part and the largest part to be Turned/machined i think i will be able to guide you further.