I also suggest getting an iPhone 3GS, not just the 3G.
I have an unlocked, jailbroken iPhone 2G which due to its age is limited to iOS 3.1.3 and am starting to notice that some companies are now releasing apps that will only work on iOS 4.0 or higher. So the next used phone I upgrade to will be a 3GS. You don't want to fall too far behind in phone model and capabilities. 1 generation back is usually ideal for a nice price point.
Even though BBM doesn't work, there's at least one text messaging app that works with SpeakOut as long as everyone you message with installs that app. I think the app is only available for Android and iOS, though.
Over WiFi only, a nice app to use which is similar to BBM is called PingChat!