What's the benefit of dual stage over single?
Should I be looking out for specific brand names?
Take a look at my guide for pressurized CO2 (linked in my signature) for more information regarding these questions.
What is a reasonable price for the regulator body if purchased used on ebay?
This is harder to answer; the demand for regulators has been increasing, while the supply has been decreasing. This results in the price going higher and higher.
I suppose it depends on how lucky you are. I saw some single stage Concoas that went for $15 just a few months ago.
I see a lot of regs that are for oxygen, or acetylene, or other welding purposes... Are those usable for this? or does it have to be specifically a co2 regulator?
In general, people either get CO2 regulators or re-purpose inert gas regulators for use.