Using the search feature will yield a lot of good information. With that being said I'll chime in with my opinion.
This hobby may be one of the best hobbies in the world to take up because the abundance of high quality used equipment on the market. A lot of people have big dreams, they get certified, buy a high end kit, and after a few years life changes and they stop diving, or the bug really bites them, and they 'upgrade'.
When looking for used equipment I'd aim to pay 30%-50% of what it retails for new, ON-LINE. Look at leisurepro.com, they may be one of the cheaper on-line stores, so they'll give you a good baseline of what something sells for new.
More of my opinion...
Buy newer, current models... this isn't because a newer stuff is better, it's because it'll be easier to sell if you quit the hobby or upgrade (my opinion).
Factor in service costs... this really only relevant to your BCD and regulators. Regulator service can run upwards of $150 for a pretty mundane service... prior owner neglect can increase this number or render the regulator junk too... Buy something that looks like it was cared for, and try to find a trustworthy person when buying (there are lots of good sellers on SB in the classified section).
All and all, I love used stuff, and dive used gear almost exclusively...