Just between us, doing liquid for over a decade
😉
reservoirs are not requirement, they (as well as QDC/ball valves) are just convenience accessory.
OP asked for custom loop within specific budget in specific country, and I posted exactly that.
regarding the process, no - filling the loop is done after HW (including rad) is mounted.
Not long ago, I did the shrink to my ATX mid tower to mini ITX while preserving the performance (OC i7 + GTX 1070).
while a single 240 rad (front mounted intake) is able to keep it cool and quite, I was not completely happy with the thermals and noise level. so 120 rad was added (top exhaust), but it there are couple of issues with this config:
1. it's ugly

2. in order to mount it, i had to remove the single piece top cover. so the other open section is sucking air in.
3. I live in hot climate, so the ambient is well above 25C for ~9 months
so obvious solution is to replace the top 120 rad with 240 rad.
but than, due to space restrictions, i have no way to mount the rad and fans inside, as well I'll have no access to the top port on the res to let the air out (not to mention filling the loop through res).
So the solution was to use 240 XFlow rad that has ports on both sides + replace the pump top+res combo with just pump top and use the top mounted rad as reservoir as well. of course I'll have to make a nice hole in the case to have an access to the port on top mounted radiator.
I'll probably will make a build log post here when the project is completely done (hard tubing ahead, probably glass).
here is the system with single rad:
https://goo.gl/photos/xjYNpaaTHS4QJFzMA
that's how it looks now:
https://goo.gl/photos/iBSNsG6APkGg2Fx18
and with a bit of imagination, you can guess how it will look later