The eBay landfinder is the best, in my opinion:
http://pages.ebay.com/realestate/land.html
With your budget, concentrate on New Mexico, Arizona Colorado, Utah and Nevada. They have the best prices.
I have my own theories on what is important to look for, but it is anything that saves money in the long run. Being close to power is important, as is shallow groundwater and water rights laws.
Find out what you need to know to improve the property the way you want. For example, there may be minimum acreage requirements for installing a septic system or drilling a well. Also, different living quarters (RVs, mobile homes, minimum floor space, etc.) may or may not be allowed due to local zoning regulations. Sellers usually indicate those restrictions in the sale, but be sure to find out if there is any question.