A fill sensors works by detecting the pressure of the water column.
The problem with a horizontal reservoir is that the water column is very short.
Additionally, the orientation of a reservoir with a builtin level sensor is important because if it is rotated so that the level sensor is at the high side, there will be no water column to detect.
It is probably doable, but you have to understand the physics involved to make it work.

Having one or two loops is more an aesthetic issue than a technical one.
If you want multiple coolant colors, then a dual loop may be the ticket, if not, the a single loop will suffice.
But it looks like a Vector Desk deserves all the bling it can get

Hopefully you could tuck the sensors somewhat out of the way.
It all depends on how creative you are and how much work you are willing to do.

If you have a single loop, then one flow sensor and one temp sensor would be sufficient.
You would also need at least one pump per loop.
A fill sensor is not really required to monitor your loop, you will notice fluid loss anyway.

That Vector Desk is impressive, but it will be complicated and you will have to go high end to do it properly.
For instance hard tubing is almost a must, soft tubing will not do it justice.
Just wondering if it may be better for you to do a regular water cooled unit first.
Not to throw cold water on your plans, and wishing you the best, but there is a learning curve involved.

Firstly, I have to retract my statement that a reservoir with a built-in level sensor may work in the horizontal position.
The sensor has to be below the lowest part of the reservoir and that is not really possible if it is built in.

Flow sensor placement/location does not matter.
As water doesn't really compress, the flow rate is constant throughout the loop.
Coolant velocity may vary, but the flow rate will be constant.

I believe one D5 pump per loop should work fine for 2 rads & 1 or 2 water blocks.

I would personally with one single loop but its up to you if you want to go with two loops. A single D5 can power all of that let alone one of the two loops.