select a pump
with a compressor type impeller rather than just a paddle pattern, as this
will prevent any sudden pressure loss in the system by back flow when a tap
is turned off and the switching deactivates the pump.

I was thinking to do something like this - can you tell us how we can
distinguish between the two?

Is is possible (or logical) to use a pump from a combi or similar
(central heating circulation pump) - the shower pumps are a bit
expensive for me (and not available here, AFAIK)

Also an idea for a low-cost pressure switch - I used an oil pressure
switch as used in a car engine sump, the only problem is to keep it
dry by placing it on the top of a closed vertical pipe, otherwise it
rusts impressively! (and run it from 12 volt otherwise you risk
shorting out the supply to earth) maybe not such a good idea after all
- flames expected!!!)