You would have to use the mps pressure sensor (fill level) with tube reservoirs. It requires to have a port at the top which has contact to the area with air.

I recommend to have a look at page 8:
http://aquacomputer.de/tl_files/aquacomp…sh_20150323.pdf

This should give you a much better idea of how this sensor works.

The number in the name of the sensor translates to the maximum height that the sensor can detect. So the mps pressure 40 should be fine for almost everything since it covers 40. Do not get the larger ones if you do not need them because their resolution is worse due to the larger range that they have to cover.

The type of flow sensor depends on the pump(s) that you use. If you want to make it easy: get the 3-pin highflow sensor. It covers a wide range from low flow rates to high flow rates and it also works out of the box while the mps flow series requires a calibration which can be tricky if you are after very accurate results.

Use the aquastream XT as pump. The Compact 600 has clearly less power. If you do not need the integrated fan controller you can also use the standard variant. The pumping power is always the same.

The aquastream XT can do 4,2 meter while the Compact 600 is about 2,8 meter. The flow rate can not be said since it depends on so many different parameters. In a normal loop you could get about 100 l/h from the aquastream while in the same setup the compact 600 might do about 70l/h.