Yes, you're exactly right; the custom editions have hand-picked chapters that the professor will be covering in class.
Buying used, cheap custom editions is the best! However, unless it's super cheap, I would wait until class before you decide to buy one because some professors might absolutely require you to switch to a new edition (or even a new book! so make sure you read the syllabus). Chances are low, but it happens sometimes, and it certainly sucks. Depending on your classes, I don't think you'll run into that many looseleaf books. They're great for portability but the resell value is like 0 lol. Full version books are a smart buy if you can find one for relatively cheap (for example, I'd rather buy a full version for $160 rather than a custom edition for $150) because selling it afterward will be super easy. Custom editions can't be sold to anyone except for other UCI students, and even then you'll have to sell it for really cheap in order to find someone.