Keep checking the used car lot. I bought a 69' Isuzu Bellet (which is surprisingly quick and fun to drive) for around 20k. It can run in both the Japanese classics and world classic events
Something cheap and rear-wheel drive.
GT1: R32 GTS-t Type S
GT2: Supra Twin-Turbo R
GT3: AE86; was uncompetitive, so I swapped it for an '89 MX-5.
GT4: R32 GTS-t Type M (second time around; can't remember what I started with on my first save)
GT5: DC5R
Judging by all of that, it'll probably be Japanese, depending on the price / performance / predicted upgradeability (the latter being where I stumbled with the '86).
Civic Type R. Now it's a RKM tuned beast![]()