Getting a brand new smith torch is only like $300. I just went that route.
However, if you are set on buying used, there are a couple things to look for.
Make sure the torch is what you want. What I mean by that is some torches are strictly cutting torches. Others are the more common torch body with attachments. The latter is preferred because they can cut, weld, braze, solder, and heat. However, if you're just gonna cut scrap, or bevel pipe or scarf welds, a dedicated cutting torch works better.
Harris attachments are only good for Harris torches, victor for victor, ect. Check and see what your local welding supply carries and see what fits you the best. I like smith because they're made in USA and lifetime warranty. Other brands that are good but are harder to find parts for include Meco and Purox.
If you buy used, make sure there are no leaks between the attachment and the torch body. You want the needle valves to close and open smoothly, all the way, and not stick, especially the oxygen cutting jet lever. Harris torches tend to stick on the jet lever.
Make sure the threads aren't damaged where the attachments screw on, and make sure the cutting tip isn't so f'd up it can't be removed and replaced.
The regulators are the most valuable part of a torch set. They are the most critical part of the torch working safely and properly. There are 2 main kinds of oxy-fuel regulators. Single and dual stage. Single stage regulators are cheaper and reduce the supply pressure to working pressure in one diaphragm. A single stage reg will tend to drop in working pressure as the tank pressure drops. This is usually not a huge deal, because the torch is seldom lit for longer than 10 minutes at a time. The main disadvantage to this style of regulator is the tendency to wear out faster. Guys tend to not use a reg properly and don't back off the pressure before opening the tank, and then fail to open the tank slowly. They crank open the valve and smash 2300 psi onto a single diaphragm and it tries to cram it down to 35 psi in a second. This is hard on even the best regs and it will eventually break the diaphragm, causing the pressure to creep up to dangerous levels when be torch is not lit, but the tanks are open. This is the most common regulator problem. You shouldn't buy any regulator where you are not able to check this, unless you're into paying for them to be rebuilt. Just open the tanks, leave the torch off and watch the low side pressure. If it creeps up over a period of 10 minutes, then the diaphragm is either partially or totally shot, depending on how fast it creeps up. If you set it for 2 psi, it should stay at 2 psi for as long as the tanks are open.
Dual stage regulators are the other type of reg, and are the more expensive of the two. Typically, you will find these in a professional welder's shop, and are usually overkill for a home shop. The use two diaphragms and reduce tank pressure in two stages, as the name implies. These regs are more durable, more accurate, and will keep the set pressure no matter what the tank pressure. The disadvantages are the cost and the size, cost being the greatest. They are larger than single stage, so they're not as portable but that's not a huge deal. They are susceptible to the same problems as a single stage, so check them the same way.
No matter what, install flashback arrestors AT THE REGULATORS not just the torch body. If you slice the torch hose, would you rather stop the tanks or the torch? It makes no sense to to just put them at the torch body.
Also, never use any oil on an oxy-fuel rig. Pure oxygen lowers the kindling temperature of materials and can cause an explosion if it gets inside a regulator. That's why they say "use no oil".
Good luck and get a good one.
