For temperature only I prefer the 18B20 sensor. They’re very easy to interface with and they can be at the end of very long wires.

Adafruit has a bunch of sensors that work over the i2c bus; they’ve included python code for accessing the data as well, which makes it easy to get started.

There are sensors out there for everything. Absolutely everything.

https://www.adafruit.com/product/1782

You can integrate just about any sensor into a Pi depending on your knowledge. (any sensor you find on sparkfun would work for someone with a little determination)

Good question.

I'm really benchmarking the system against itself, year on year, by comparing the weather outside vs. performance (output temperature of the HVAC unit, average temperature inside the house).

I'm not trying to find an objective efficiency rating so much as monitor performance over time to see if it's changing.