The correct sensor depends on the pump you have and the number of CPU/GPU water blocks in your loop.
One way to find out is to install the Tee connector on the outlet side of the pump with a LONG hose on it. Run the hose vertically up to as high as you can make it reach and put a bucket under it just in case. Then start the pump. Use a ruler and measure the difference in hight between the water in the long tube and the level in the res. Convert to millimeters, select sensor, install, done.

Correct choice is item #2. Sort of.

The total head of 27.9 feet is the specification for the pump with no flow, ie. the outlet port is plugged.

I do not know what pressure you want to measure, but if it was me, I would measure the pressure difference between the pump outlet and the last CPU or GPU cooling block outlet side, or after the filter if that is the last thing in the loop.

This will be much less that 27.9 feet with flow through the Loop.

But the above is for measuring differential pressure. If you only want to measure the pressure at a specific point in the coolant loop. just use the hose connected to a Tee fitting and measure how high the water goes. I do recomend a bucket under the hose though.