300mm Ai/Ais?? which particular model?

There's an f/4.5 model currently for sale in the classified somewhere here??

Not 100% sure on the absolute quality of that lens, especially with the 2xTC but form memory the most reliable source of info on Nikon specific lenses seems to think they're not too bad, as long as it's an ED lens. ED is of course the type of lens elements.

You're in Melbourne:

two things.

1. if you can find the time, try to make it down to the Melb meet coming up in a few Wednesday nights, and you can have a play with the Nikon 500 mirror lens I have.

2. if you can get into the city, check out Camera Exchange on Londsale St half way up the hill between Elizabeth and Queen Sts, and opposite JBHiFi.(number approx 380-ish???) I recall them having two copies of Nikon's 300mm f/4.5 in Ais mount, but can't recall the price they were asking.
Their good blokes in there and will allow you to mount the lens to your camera and take some sample shots. They also had various teleconverters for sale as well. May just be the medicine you need