Thanks! Oh, and don't forget the environment-controlled waffle maker, its clutch
So I'm actually using the Atlas Scientific Tentacle Shield Mini with one of the Atlas pH control modules (I'll eventually get other modules, so sprung for the multi-component shield). I just did the I2C wiring to the Pi instead of the USB module. The USB module gives you the... UART interface- all the atlas stuff can do UART (over USB or direct) and I2C. I mostly tried to use I2C interfere for everything due to lack of UART ports, but the USB connection should give you an 'extra' UART. Should work great still.
That was about $100 right there. The probe itself, I got a Milwaukee Instruments probe from amazon, about $40.
The atlas stuff uses the same BNC probes that everything else does, so any pH probe with a BNC connector should work. I went with a cheap one, but not the cheapest- the chinese ones apparently drift really bad and need to be calibrated often. The Milwaukee ones are good industrial (but not lab) grade probes. I'm shooting for being able to run the probe a full grow without calibration- a decent one should be able to do that for you.
Thanks for the info, about to get one going I think!