Three digital pins is what you need for the EC sensor. Temperature sensor is separate - can be analog (NTC, TMP35/36) or digital (DS18B20), doesn't matter really, as in the end all that's really important is the temperature value.
The I2C version is a very different animal. It's to be built around an ATtiny, so all you need is the connection to the I2C bus, with the ATtiny taking care of everything else (reading the probes, calculating EC based on the measurement and calibration data, temperature compensation). That would probably include an NTC temperature probe, and maybe even a pH circuit. I have the circuit designs and the parts, just not had the time to actually build and test it. It's a long term target.
ReverseEMF:
It's still a voltage divider, but the current source/sink behaves like an infinite resistance. And, because the current remains constant, it can, perhaps [because I have no experience with sensors of this sort], be set to a level just below the electrolysis point, and it will stay there over the whole usable range.
Electrolyses starts at about 1V, I forgot the exact number. It's just part of the problem. The other part is the migration of ions due to the current, the positive ions cluster around the negative pole, the negative ions around the positive pole.This causes changes in the resistance, and is the primary reason to use AC: the constant changes in direction prevents ions from moving. The minimum frequency is 1 kHz, better go for 3 kHz or more. On the other hand over about 300 kHz, and certainly over 1 MHz, it doesn't work well any more. I don't know what happens, but the conductivity goes down. The highest frequency I've reached in testing was some 950 kHz, and that was for a 3.1% salt solution (a little above seawater). The 555 I use can reach 2.7 MHz, and the ATtiny can measure up to 4 MHz (it's running at only 8 MHz), so there's plenty of room 