As mentioned, the factory block is the weak link and it's hit or miss on when, or even if it will break. I know cars making 400 hp that split the block in a month, and I know several cars making 550-600 that have been running for years.
Best bet is to start with good stuff. And if you've never put an engine together, might as well start with a crate motor. Look at FRPP, they have all sorts of power levels available. Check out Craft as well. You're going to be over you $5k limit pretty quickly, no matter which way you go, so might as well leave the assembly to the pros.
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