70s - Nissan Bluebird 1800 SSS '79. Buy one used for around 5,000Cr. Should win most races stock, most common of the used 70s cars in the historic lot. Mitsubishi Lancer 1600 GSR '73 is a better bet than the Honda Coupe IMHO, since it has the same power, but is FR unlike the FF Honda.
80s - as hispeed said, the first 2 of those 3 should do nicely, very cheap too. 👍 The Non-supercharged MR2 I'm dubious about though, without modifications it's not fast enough to keep up.
90s - Miata was very tricky for 200 points stock, MR2 is more powerful and could probably keep up a little better. Otherwise a used Honda Prelude, the '96 SiR is a a good bet for about 160-ish stock points.
With the cars mentioned here, you should be able to beat all 3 championships for under 40k, nevermind 250k!