can the OSPi do weather monitoring via a hardware temp/humidity and maybe rainfall sensor?
The current version of OSPi has a rain sensor terminal and you can connect an off-the-shelf rain sensor to it. These rain sensors are basically rain-activated switches but do not measure the amount of rainfall.
Temp/humidity sensor is possible. You need to wire the sensor to RPi and modify the code suitably.
There are also off-the-shelf rain gauge. I’ve recently looked at a wireless rain gauge and wrote an Arduino program to intercept the signal:
http://rayshobby.net/?p=9056
I haven’t adapted the code to RPi but it should be fairly simple.
Can I do datalogging and run it as both an irrigation controller and a weather monitor?
Dan’s interval program already supports logging and I assume it shouldn’t be difficult to modify it to log weather information as well.
7 core 0.5mm2 wire with them
I am not an expert on this. You can check this page about wire thickness vs. distance you can run:
http://www.hunterindustries.com/support/what-maximum-distance-i-can-run-wire-between-controller-and-valves
I am guessing OSPi also has full wifi?
Yup. Technically it’s Raspberry Pi that has WiFi — you can use a nano-size USB WiFi dongle. That’s a pretty standard addon to Raspberry Pi.