I think that 2 classes is great. However, if you're going to have a spec class, run the limit at $250 or $280. Initial cost is higher, but in the long run you're going to have an engine that runs much stronger and longer. The $200 engine rule just means that you end up having carb/bearing leaks or the engine is worn out after one season. Then after the $280 price point, run an open class option.
If they're is a low turn out in an area. Still run the spec/open rules, run the cars at the same time on the track Le Mans style. But just score them separately. I know that sometimes that can be discouraging to new drivers, but at the same time it allows the new guys to follow lines of faster guys and see how they get around the track and mimic them. I know that's how I fine tuned my driving when I started was when I got passed by a faster guy I would try to follow him and stay on him to learn the lines around the track. By running the spec/open together guys can run any big block they have on hand, build the class overall, but still give the spec guys a chance to win. Then once the class is built up enough you can split them into 2 separate classes.
As always, my 2 cents, so take it for what it is.