For a first stab I would go with soft tubing and barbs. Barbs are much easier to handle than compression fittings in my experience. So much so that I went back to them when I recently redid my loop after an upgrade.
My limited experience of rotaries is that they leak if under any sideways pressure. I hardly use them any more, I have one on a straight vertical drop from res to pump.
The drain valve (koolance) I had must have been too narrow, it was barely a trickle. I can actually swing the entire Res/pump part of my loop out of my case and I found it easier and faster to just undo the pump outlet.
I wouldn't bother doing a GPU, you can get decent air coolers on factory oc'd boards and I've never found mine to be thermally constrained to the extent I got any significant advantage from the cooler temps. I found it a royal PITA fitting the blocks, and even more annoying when you want to upgrade GPUs.
For flushing I just run mine through hot water in the bath! Can rinse with distilled or whatever afterwards if paranoid (I didn't bother).