First off, there is little likelihood that you will find any suspended bike for $600 that is going to be worth the money. A5 that price point you get heavy shocks with little control that would probably make the bike less, not more nimble.
You can go to BikesDirect for lower-cost bikes of all kinds (I haven't checked the site in a while but prices used to be okay.) Or, go to any major manufacturer's website and see what they offer. Learn about things like "rebound" and lock-out" and such. Look at how much travel different forks/shocks offer. Decide what you need.
Do you plan to launch off ten-foot drops? Do you want to ride mostly smooth trails with some roots and rocks and ruts? Do you want to go really fast? Do you want to go downhill at 40 mph over boulders and logs the size of people?
Bike suspension is so advanced nowadays that you can get a bike which will let you ride terrain your sill alone will not let you ride (ask me how I know.) But .... to get the good stuff you either need to pay the big bucks or get incredibly lucky online (FB, CL.) You can find great deals on lightly used bikes online.... but remember mountain bikes by their nature tend to get abused, and I don't care how good a shock or fork is, if it hasn't been maintained it can blow a seal or something and leave you stuck. You need to really know what you are looking for and when you go to look at a specific bike, you need to know what you are looking at.
A key question others have asked is, "Do you need full suspension?" I am not judging or even suggesting---I ride F/S because I don't have the skills I once had, and I don't care. But ... it is your choice. What kind of trails do you want to ride? How do you want to ride them? If you want to bash your way over obstacles with zero technique then F/S is an option. If you plan to ride really gnarly trails F/S is an option. If you are a more mo0derate rider who wants to put in a little effort t develop some riding skills, hardtail is lighter, simpler, and a lot cheaper.
Whichever route you choose, do your research. Decide what you want to ride, and how, and learn about what is out there.