I think you’ll find that a $20k budget will soon blow out, especially with freight and the exchange rate. A pair of complete Pond heads will end up over AU$10k alone. New cams are US$880 each. A set of T&D rockers cost me over $3k. It all soon adds up.
You have to be pretty committed to wanting a SOHC. There are certainly cheaper ways to make big FE power, especially with Jay’s top end kit.