Tubing can be summarised as size doesn't affect flow rate, normal choice is 7/16" over 1/2" barbs with jubilee clamps. Tygon is worth it for the bending radius, coloured tubing with deionised water much better than clear tubes with dyed liquid.
Compression fittings are basically barbs with jubilee clamps built in, they're pretty but prone to tube twist and gradually winding themselves out of the blocks. The tube size has to match the compression fittings exactly, 1/2" tubing with 1/2" barbs or 3/8" with 3/8" etc. 11/8mm is difficult to source pieces for.
The D5 is excellent. I'd want the ek top for it, in order to use the same barbs with it as with the rest of the system. I don't trust plastic barbs. The DDC is the other option, again with a top, but either will do you proud.
The reservoir is convenient for filling but not required, a T piece with a length of tubing and a fill port/valve on the end achieves slightly lower temperatures and a system which reacts faster to changes in processor load as there is less water in the loop. Connect it just before the inlet to the pump. A second one to empty the system is a really good idea if you don't want to have to turn the case upside down to get the liquid out, I've run a line down from my graphics block with a valve on the end.
This thing is threaded both ends, I've got a blanking cap screwed into it as well as the valve closed because I fear leaks. Can do both fill and drain lines for well under £34, gaining more space in the case, better temperatures and the ability to drain the system. The drawback is that it takes longer to fill as you have to repeatedly pour water into the line rather than fill the res and wander off.
Any questions? I knew you'd see the sense to pumping water around a computer given time
edit: I wrote this without seeing the above by Plec. He's right though, I'm still keen on copper waterblocks, and even posted an enticing photo. Here's another, I was once teased for describing this as beautiful.
Water works so much better than air does, can't imagine going back to the saner option. Better temperatures, less noise, so much more control. Brilliant.