In my build I'm using 4x Gentle Typhoon AP-15 and I mounted the fan that came with my Corsair 350D behind the PCI slots to pull out the heat my R9 280x DCU2 generates. It's still on air, but in 9 days I'm gonna fly home and make it wet
I bought my fittings for 5.50€ each, that's about 7.60$ from aquatuning.de (I'm from Germany, but living in Asia). They are made by aquatuning (kinda house brand) and therefore one of the cheapest fittings available. I'm also using lots of 45° and 90° adapters, all my tubing will go straight. They do have a impact on flow, but I'm using a single d5 for a tiny loop so I don't care
->compression fittings are pretty expensive, you can also have a look at barbs (I would secure them with zipties) they should be cheaper. I paid 50$ for my 7 fittings
When you expand your loop, I wouldn't use a 2nd res/pump/rad combo, I would go with a standalone radiator as you won't need another pump. The combo is also 72mm thick, that's nearly as thick as a monsta (80mm) radiator, but the radiator itself is rather slim, check again if you can even fit 1 of them. I would use smth like this
http://www.frozencpu.com/products/14917/ex-rad-335/Alphacool_NexXxoS_XT45_Full_Copper_Dual_120mm_Radiator.html or if it fits the thicker ut60. It still might not be enough to get great temps with 2gpu/1cpu though... You could mount another 240 to the back of your case, will probably look strange but it would give you 720mm of radspace, which is more than enough.
Some say 120mm per block + 120mm more are enough, but this doesn't incorporate the thickness. If your spending so much money on water cooling, I wouldn't be happy to get bad temps because I got not enough radiators