put the WRX STI engine in the BRZ. Still, getting a new vehicle that performs like a slightly modified Z32, will cost way, way more than 20K in today's market. Buying a worn Z32 and restoring the driveline and suspension cost 20K but you still have a not-so-nice paint and interior.
There is no way around spending lots of money in order to get a nice surface and interior, attached to 400HP, with good styling and sport suspension. 100K maybe?