The start of something new

To run alongside my other big build project (slimyroadster) I've decided to start on a smaller project that will take knowledge from the car but no doubt have new challenges of it's own. I'm going to try and build.... a desktop CNC milling machine.

The rough requirements are as follows;

1. Desktop sized. By 'desktop' I mean the workbench space that is currently taken up by my arcade machine, which is roughly 400mm square.
2. CNC controlled. Pretty obvious this one, I want it to be able to create computer designed 'things'.
3. Strong enough to work with aluminium. This will be a tough one, the general consensus is that being able to mill aluminium is not cheap. 
4. Able to host a decent sized cutter. I'm thinking something like this, although that is just from a quick Amazon search. I'll do some reviews. 
5. It needs to have the option of changing cutting tools fairly easily (not just the cutter, the whole tool). I suspect that the first thing I'll be updating as I go.
6. Along the same lines, the motor mounts also need to flexible. Not flexible as in able to move, but flexible to allow the motors to be upgraded in the future.
7. Metal frame with a moving gantry. Note I said 'metal', not specifically aluminium or steel. I'll no doubt add a dedicated post around this as I have a few thoughts in my head. 

So that's where I'm starting. Oh, and one more thing;

It needs to be cheap!!!