Using the moisture sensor in a way that would allow it to get repeatedly wet would (assumedly) result in it getting scummy and failing to read correctly. I don't think it's a good long-term solution.
With respect to what Arduino you need your primary requirement is that you have enough IO pins for your project as it certainly isn't a processor-intensive task. There's 14 digital and 8 analog on a typical board. A Mega will give you more of both or you can add on an IO board like the Mux Shield
. Whether or not you use multiple boards really depends on whatever you think is practical -- taking into account that you might need to create communication between them which could create headaches in itself depending on the distance you need to cover.
If I'm simply setting:
An Archimedes-Screw on a servo motor set to spin for 1 second and then shut off for 4 hours.
Thats what? a servo attached to maybe 2 pins? If that?
electronic PH tester to check for PH drift and changes once per hour and report to a Linux PC.
I have no idea how I'll wire an electronic PH tester to an arduino....Still, probably not the best idea to keep this around. I may have to just drop features until I get better equipment.
the room lights to allow for the grower to see should be activated, simple t5 tubes (aprox. amount of tubes: 2-4)
I'm thinking I'm going to drop this, as if this guy is too lasy to hit a light switch I just dont know anymore...
a moisture content sensor to detect overflow would not be getting wet often, if it did then my system would be an utter failure, as a matter of fact, they should remain dry unless a freak incident occurs and the main draining area get clogged.
I'm thinking one board will do, with control over a servo, and moisture sensing. I may very well pair off the T5 tubes to a simple light switch, I mean really I dont think it's needed.