These days most smartphones have WiFi, a camera and GPS, so that part is easy. Also, most allow you to turn off cellular data so you can stick with WiFi. It's not cheap to get an unlocked iPhone 3GS or 4, but they would fit the bill. So would Android phones, though again you'd have to pay retail via an importer to get an unlocked one (around $500). If you start a contract, which I understand you might not want to do, our US carriers will generally provide an unlock code for contract phones after 2 months of service. In this country, you want a phone that's GSM, and that means T-Mobile or AT&T.
If you're looking for a less expensive or used phone, there are Nokia S60 smartphones that are sold unlocked for less.
-------------------- Lisa Gade Editor in Chief, MobileTechReview