Well, since I did not yet get my hands on the case and my spatial imagination isn't the best, I won't make any final decisions before actually building my first system in a C4
Usually I prefer longer tubes over having additional connectors in the loop.
Of course there are huge price differences
EKWB, Watercool and Hardware Labs have a reputation of having really well built stuff (manufacturing wise), but personally the look and feel of Alphacool products is sufficient to me (when it comes to radiators). And yes, you can totally use XSPC, keep in mind their slim rads have a higher FPI (fins per inch) count than ACool for example, so use a fan with high static pressure (which is generally adviseable on radiators anyway...)
Just make sure to choose only 1 material that comes into contact with your cooling liquid to avoid heavy corrosion debris.
There is a post somewhere in this thread talking about max. GPU block dimension, feel free to search for it