Which would be the best s30 model to invest in?
An investment strategy is something you talk to your financial analyst about.... These are a passion. That said: Early model production seems to drive the high $$. Those are the ones I would feel bad about major modifications, so buy one of those that was recently restored - you can probably buy it for 1/2 to 2/3 of the cost of the restoration. Keep it for 5 years, maintain it, and you can probably make a few grand. A lot of these cars are worth more in parts than as a car, so there may be money in that area.
What seems to make the most money? or get the most attention? Is there a good year or engine size that came out and everyone went 'That's the one to have'
They all look very similar (bumpers and mods are the exception). If you can get one of the first 1000 produced, go there. They will continue to appreciate.
People make connections with cars... In the 70s, US cars lacked an identity, and were junk. Some of the Japanese cars filled a void left from the muscle car era. These cars may appreciate slightly in value, but there's very few "Mach 1"s, Ellenor, Hemi Cuda, etc. models and they are locked away somewhere.
So, find a car you like, play with it and maybe you'll have fun and won't lose much money. Maybe you'll get lucky and make 20%. If it's about money, don't do it.
Just my opinion.
Mike