Design help etc required with DIY CNC Router Design / Build

[Jonathan]

I am about to source my bits 1605 ballscrew and nuts, bk/bf12 etc

From Chai / linearmotionbearings2008 on ebay?

Using supported round rails 350/750/1150 on heavy gauge 40x80 aluminium profile. Is 16mm enough or should I go with 20mm for the round rails with the linear slides?

I definitely wouldn't use any less than 20mm rails. I currently have 20mm rails on Y/Z (900/400mm) and 25mm rails on X (2000mm). The 25mm rails do seem better, i.e. much more rigid with less play. This is no doubt partly due the 25mm bearing blocks having one more row of bearings, which evens out the loading a bit and is obviously stronger overall...
Theoretically the load ratings for the 16mm rails are plenty, but in practice people use more. Bear in mind that though the rail is supported that support is not perfect - they can flex upon the support, but only in one direction not all directions like with unsupported rails.

I should also mention that I do have 15mm profile rails which I will soon fit to the Y and Z as they are clearly much better.