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:
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:

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.