the UNO is the defacto standard to start with. the number 1 reason is that the shields are made to plug in and then work without having to make circuits.
in your case, you will need some cirucits, so the NANO or Micoro, the ones with the on-board USB chips make great sense.
first, if can make a compressor that can generate the pressures you needs and you can turn it on and off with a switch, you can replace the switch with a relay and some control of the Arduino.
if you can install a pressure gauge, and then feed that back to the Arduino, the arduino can use that as the reference point to create the on/off signal for the pump and valves.
add a selector switch for the level you want, feed that to the Arduino and presto, you have a complete system.
couple relay boards from e-bay are cheap.
a NANO or MICro from e-bay, easy and low cost
250 mmHg or 3/4/in W.C or 0.35 PSI is very small for many devices.
do you have a sensor that has a full scale to around 250-300 mmHg ?
there are many manufactures that make these low pressure sensors. much better to get one that is in this range than to get one for a much higher range and then only use a fraction of the range.
Peristaltic pump can generate these pressures, but not high flow.
an air brush generator can as well
I think a decent fish tank pump can also generate a couple PSI so they should be able to do that a well.
my experience is that a pump will deliver a very noisy signal. every stroke is similar to a small fire cracker going off inside of the vessel.
I used a pressure regulator, a capacity tank, then a second pressure regulator, in an effort to remove most of the fluctuation caused by the pump.