Buyng 2nd hand is risky if you dont know the machines history. If you can get a demo of the machine, and its working fine, then you know youre at least getting a working machine. However, things do break and with a 2nd hand machine you dont know when that will be.
If youre comfortable working on your own machines, then 2nd hand is a great way to go, as espresso machines arent too hard to fix. There are a lot of tinkerers here that have gone that route. However if you just want to buy a machine and have it work, without any hassles, then I suggest buying new. The price you save on a 2nd hand machine can easily be spent in expensive service/repair costs.
BTW: The Gaggia Classic can make very decent espresso, but steaming milk is poor. Once you steam with a HX machine youll appreciate this.