The issues with the smaller mills come down to stability and capacity.
Stability is how rigid the whole thing is. As you can imagine, vibration can affect the quality of the work. Even the largest machines have vibration, but rigid/heavy/strong enough means the effects are small.
Capacity is how fast it can remove material. A big machine might be able to remove a cubic inch of aluminum in a second, while a micro mill might take a minute or two. This is only a question of time, not quality, so if you have time but not money, a micro mill's lack of capacity won't be an issue.
That being said, the micro mill seems to be at a premium because of the CNC control. Note that anyone who can build an FRC robot can absolutely retrofit a manual mill for CNC operation. The 'problem' is not one of software or electronics, but merely attaching a motor to each hand control Stepper motors are commonly used).
For a full-size mill, you'll need 3 or 4 axes of control, with relatively large stepper motors. Such a system is under US$500 before shipping (
Link). Then you need software (Mach 3, under $200) and a computer (maybe free, Pentium 4 is more than enough). Add maybe $300 for brackets, cog belts and cogs, limit switches, etc. I would imagine that all of this is available locally, it is not all that high-tech.
That doesn't include design software, but I don't see that as an issue.
If you really don't need 3-dimensional machining - essentially, you only cut out shapes from flat sheets - consider a CNC router.