when it comes to spindles, it doesn't matter that you have a small machine.
What matters is what you want to cut with it. Also, thinking of your woofer holes, you need to design in enough bed room for your sheet material, not just the cutting area.

I would say 300w is quite underpowered for anything except shallow engraving.

The larger spindles, and the water cooled ones are VERY heavy ! this will have to be considered into the design and motor/driver choice.

basicaly, lenght of screws/drive/r&P etc and length of rails/bearings determine the machine cost, (unles using extruded ally which is expensive). Most of the rest is the same cost regardles of machine size.