This really depends on what you want to do. For a general data acquisition card a National Instruments E-Series card would be appropriate. I think the cheapest E-Series card is a NI 6024E. It has 16 single ended analog inputs, 2 12 ananlog output, 8 digital lines, and 2 24-bit counters. This with a copy of LabVIEW (and package, base, full, or professional) is all you should need to get started developing a data acquisition system. One additional feature that you might want would be a breakout box like a scb-68. This will allow for easier wiring to the DAQ hardware. I am not aware of any program that lets you buy used DAQ equipment. For more information on specing out a system go to http://www.ni.com/advisor/