You can find good deals buying used stuff, but make sure you know what you are getting. I see a lot of people trying to sell a package of stuff for $600 or more, saying they've put a grand into it. They may have put a grand into it, but if they made poor choices with how they spent the cash, it doesn't do you any good at all.
If buying new, here's what I would do:
New AX10 kit - $210 (
www.rpphobby.com)
used Spektrum DX3 $100 -$125
new Hobbico CS170 servo - $90
1300 mha, 3S Lipo pack - $35
Celectra charger - $25 (ebay)
motor - $20
new Tekin FXR ESC - $90
Panther Cougar tires - $36
So for $600, you have good quality stuff, and a good base platform to upgrade with things like bent links, aluminum stuff, dig system, etc, etc. If you can fabricate a new skid plate, and get creative, you can do quite a bit with the stock AX10 geometry to get it to perform better.
You could buy a cheap AM radio, and a cheap servo, but those are things that you will end up upgrading later when they let you down.
If you are buying a used setup, make sure you're getting better stuff than I listed, or negotiate a better price.