I bought my Technics and the DJM second hand and they're both thoroughly unbreakable (although I only just broke the headphone socket on the mixer yesterday - we were in fact close to disaster lol)
CD decks I think I'd only ever get new though, they seem a lot more fragile in comparison and strike me that they probably will need replacing at some point before long.
My only advice would be to learn on old/cheap decks, but when you're making your "big" purchase, save up and get the better stuff because it'll just last longer. To be honest though, none of it really matters anywhere near as much as simply practising