You really want something that is at least 2HP, 26-30 gallon tank, though a 60 gallon tank is better for the air ratchet, and at least a 105 cut in. Some compressors come with 90 psi cut in that are non adjustable. With a 90 psi cut in, by the time regulator and hose pressure drop occurs, your tool will only be seeing 75-80 which will cause tool performance to suffer. In that case you can change the pressure switch. Craigslist prices can be all over the place, but $300 is a nice target. Air ratchets typically run for longer periods of time than impacts which is why the larger tank is helpful. These will not run air drills or die grinders for any length of time, but for basic impact and air ratchet use they will be fine. When buying used be wary of any older tanks that are rusty and or leak. You cannot safely repair a leaky tank and replacing the tank is generally more expensive than the compressors initial purchase until you get into expensive compressors. Buy an oil lubricated belt driven unit because they are much quieter than oilless. The oilless will be much cheaper as most folks selling them have upgraded after they couldn't take the noise anymore, or needed something bigger.