How about fine tuning testing w/ Davinci and then scaling it down for the other models or HF once you've proven it works. I believe the openai docs propose this approach (minus HF of course)
This works well. Starting with bigger models allows us to explore whats possible. I find it easier to scale down from here.
That's up to you. Many don't want to open an account and pay in order to explore what's possible. There are LLMs available on the HF Hub, such as google/flan-t5-xl.