I really wouldn't try to keep more than a couple of fish at a time as a beginner. And this is one time when buying used should not be a problem. If the tank is cleaned thoroughly and the lid fits well, then a new aerator/pump should be the main investment. I wouldn't buy a used aerator. Sometimes, they don't work very well.
This is also the time to consult a friend who keeps fish on the art of doing so. They don't take a lot of stress very well, and can get sick easily if their tank and water are not just right for them. So, do yourself a favor and get someone who knows about keeping fish successfully to help you pick suitable species and set up your tank for them.