Just simply too vague of a question. ( I realize you know that already).

Realistically, your better saying, "here is my budget, here is what I want it to do, and what can I get for that"? the 500-600 HP range is just too wide of a swing, and so many things can change within that window. A simple stockish 440 with wsome upgrades, and something like Sidewinder heads can be done fairly reasonable, if you want stock rods, hyper pistons etc. Again, I think you can back into easier than trying to figure out how to buy something for 600HP. I should add, i have seen a lot of what seem to be good deals out there this year, but be prepared, none of them are EVER as they appear.Just my 2 cents.