[jocko]

HO usually means more hp than the base model that the place offers. GM offers some HO designated motors that have upgrades in component performance levels as well as "longevity" assuming it's gonna be pounded on. Cast vs Hypereutectiv vs forged pistons, roller vs flat tappet cams, induction system/cfm, etc. HO for GM usually means vortech heads at a minimum, but hp/torque levels of their different offerings usually increases directly proportionally to price.

To your last question - yes, you could build it yourself, but then you miss out on the engineering that the factory has put into its proven combos. If you WANT more hp, etc, then a HO crate is a good option. If you want to build old school - might not get the same bang for the buck (both hp and reliability) doing it yourself. My 2 cents.
Having sad that, I prefer the non-crate route and building what I've wanted in the past - so it certainly comes down to pers pref.