i like hard line for their aesthetics, but one thing about HL hit me, if there's an issue, ie i want to remove heatsink and re-apply the TIM paste, system has to be drained and hard line tubes dis-connected. Soft line i can push the heat far enough out of the way to clean it and re-apply
i will use hard line for the drain line going to the ball cock.
Only reason i asked about what case, every radiator from different mfgrs have different dimensions, ie no two 280mm rads are the same in external dimensions. My Fractal R6 had a linitation of 147mm in width that was hard to live with in the 280mm rads that were 38mm or less in thickness,. FInally found Alphacool had one.
With a PC desk, you've probably got a lot of freedom on dimensions
as to using clear distilled water, you might want to take a look at what EKWB said in discouraging that
https://ekwb.zendesk.com/hc/en-us/articles/205235141-Antimicrobial-algae-protection-
there was a pretty decent 3 part blog post EKWB has up on liquid cooling, but for the life of me, my google fu isn't working.
UPDATE - just found part 3, parts 1 & 2 should be referenced in it, so you can go back from there - worth taking a look at
https://www.ekwb.com/blog/radiators-part-3-surface-thickness/