Hi and welcome.
Firstly if you are new to the hobby don't buy used. You will almost certainly have to strip and rebuild and replace some parts.
That said don't buy the cheapest new thing you can as you will suffer from quick wear, parts supply problems etc.
IMO the minimum entry level into this hobby is the hyper 7. You should be able to get one around $300. They are super tough, cheap and parts are plentiful in most of the world.
These questions are asked daily so a quick search should give you more than you can possibly read on entry level buggies.