Welcome to the BBForums.
You can get any GSM network device, unlocked, to use on TMo. Any AT&T/Cingular device will do. How do I unlock my BlackBerry? - BlackBerryFAQ
WiFi is not the same as EDGE. You will need specific model BBs that include WiFi access, such as the 8120, 8320, 8820.
You can buy a used Pearl for $150, and the 8700 for under 100.
Good luck. Post back with other questions. Check out the Buy Sell Trade section here, some reputable dealers here. Be wary of eBay for BlackBerrys. Buying a BlackBerry off eBay - BlackBerryFAQ