Two key issues with buying a 2nd hand bike:

1) Make sure it fits you properly. If you buy a new bike from a shop, the bike store has the responsibility of properly fitting the bike to you. If you convince yourself that a particular used bike is the one you want, you might disregard the hints that your body is giving you that it might not exactly fit you. And then you're out of luck, and will probably end up selling the bike at a loss before getting one that's properly sized for you.

2) Make sure either you, or someone you know and trust, can perform minor repairs and adjustments and maintenance on the bike. If you buy a new bike from a store, they will do this at least for an initial time period to make sure everything is set up properly.

If you are comfortable that these two criteria are satisfied, then a higher end used bike will offer better performance and value - but as a complete novice, the odds are that these two criteria will NOT be met when you buy a used bike.