Order of loop won't matter in any way as long as the pump is always fed with water.
Stick to a single loop as you can then use the whole cooling capacity of all the rads for all the components, believe it or not going for dual loops is usually reserved for aesthetic reasons only and not performance.
As far as the rads are concerned - the more rad area then the lower the fan speed you can run. As a bare minimum you require 1x 120 size rad area for each device [CPU/GPU etc] plus an extra 120 for a bit of headroom. So for 1 CPU and 1GPU you would require a 360mm overall rad length as a good minimum size. Anything more will allow you to run fans at lower overall speeds which can result in a very quiet PC if you have enough rad space. So the AX120 + AX240 will achieve this minimum - Adding the low profile 280 is a bonus if you can fit it in without too much trouble.
In your case, I would aim for at least 420mm worth of overall rad length if utilising 120mm sized fans as a minimum.
If you overclock/overvolt your CPU and GPUs then much more heat is dumped into the loop so I always try and opt for as many rads as I can fit comfortably within the case.
Keep away from bay reservoirs - they are a pain to fill and bleed and are generally a lot noisier.
IMO - Go for the D5 XSPC Photon glass tube res combo. Just be careful of its size as the 270mm is just the reservoir length. The pump adds a fair more to its overall length otherwise go for the shorter version if the 270 will not seem to fit. They are a good looking res when illuminated.
Here is my rig with a standalone Photon 270 [using separate Swiftech MCP35X2 pump]. Coolant is an automotive glycol based [5%] formulation which is red - Diluted with distilled H2O.
Case is a Corsair 900D - So you can get an idea of the length of the Photon 270 without the integrated pump...!