FWIW I'd go new, especially since it sounds like you don't have bike repair skills yet.

If it's an older bike, you might get lucky and get a bike in excellent shape for a good price. You could also get a bike that was poorly maintained and will turn into a money pit as you fix things.

New bikes from a real bike shop will also usually come with a year of free basic service, as well as at least try to fit you properly. If you buy used and you don't know much about bikes, getting something that fits is another crapshoot.