Xavi!

Firstly, welcome!. My apologies for not replying sooner, I did read it yesterday but suspected others would have responded by now.

Most peoples requirements are generally the same, if you read a few build logs, most people say "I want to cut wood mainly wood" then drop the bombshell "but a bit of aluminmium aswell". After a while as you read through the build logs they begin to realise the machine spec difference required between the two target materials, What Im getting at, is that its hard to have a machine that does both very well.

I assume, by stating the length of the machine is 2500mm (2.5m) you intend to use sheet stock (ie sheets of mdf or ply)

I myself, have had two 8x4 machines. One of them was a stock "production" router, running on rack and pinion.
http://www.mycncuk.com/threads/6166-My-acquisition

The other was an upright one I built myself (similar design to what you have proposed)
http://www.mycncuk.com/threads/8242-...ght-ply-cutter

Both of them, could potentially "have a go" at milling a bit of alumiumum. However, Not very well.

I think your design looks about ok, maybe a bit more triangulation of the frame (you'll be suprised how much the thing dances about, and flexes when its moving)
If your going with an A frame gantry like you have proposed (id space the X bearings further apart. Id maybe even think about making the gantry itself a different shape to promote vertical forces.

The other thing experience has taught me, is dual steppers for X is a pain in the arse, you really want one motor joined to both sides (with ballscrews, this can be achieved with a big belt) (most recently neil's (njhussey) has adopted this technique) however your saying rack and pinion...
I intend to build another machine next year, specifically for cutting 8x4 wood and plastic. I intend to use rack and pinion on both sides, -----drive one rotating shaft that runs the width of the gantry, powering pinions at each side of the gantry along racks... I can draw you a picture if necessary

your budget seems a about right....the yellow one, (used cheap linear rails, and cheap electronics) and cost me £2000 in parts. alot of that was the dual ballscrews and rotating nuts.

Others will have different opinions... read a few build logs, post us some more pictures of your design...