Edit: I suppose this is punishment for posting so early. I have since read that typically folks would recommend the HRG20 size over the 15s for several reasons, and that in fact via Aliexpress, even with import, they're pretty affordable. Although the supported round rails are even more extra affordable!
Hi and welcome to the forum. You have answered you're own question in that 20mm rails are preferred because of the spacing ( the 15mm are just to tight to get in) If you are want to cut ali with any repeatability you will need square rails all round.

Don't buy any motors, drives of any electronics until you have the frame sorted.

So as usual try and get a design together using sketup or whatever. and the good people on here will guild you through the maze. Good luck with the build. ..Clive