Hi John,
To my limited knowledge, you won't be able to find a new astro camera with this size CCD sensor for 2000 AUD; I will be extremely happy if someone proves me wrong...
The dimensions of a common KAF-8300 sensor are 17.96mm x 13.52mm, which is bit smaller than what you already have, but at F5 you will probably get some vignetting with this CCD anyway when using 1.25 filters and external filter wheel. Anything larger will cause more severe vignetting in combination with 1.25 filter.
Perhaps you could consider a smaller sensor or a second hand camera with 8300 sensor could be a reasonable option?
I am sure more knowledgable and experienced imagers will soon provide you with a thorough answer.
Good luck
