If you can afford little bit more on the rails then get the 25mm (at least for the X axis) as it has one more row of balls than the 20mm rail. On my machine I used 25mm and 20mm and the 25mm clearly has less play in it. Might be a bit over the top for an A4 size machine...

So it's just aluminium box section, not aluminium profile? It must be for that price...if it is just box section then how are you intending to fix the ends together? Steel+welding would be easiest.

Have you considered getting one of the water cooled spindles that are available on eBay? I was going to get a Kress but decided/was persuaded that the water cooled one is better as it's quieter, less runout, stronger in general and won't wear out as fast. The collets are also a lot cheaper than Kress. I got the 2.2kW version since it takes 1/2" collets.

Have you got a milling machine to cut the 10mm aluminium? If not I can help you there ...