Most apps are still written for 1.6+ (I know of at LEAST one app that will NOT run on 2.x+, but WILL run on 1.5/1.6).
As for the data plan, the Behold II and the Nexus One both have/had 3G connectivity on a regular $6 or $10 internet plan. No Android Data Plan required (Regardless of what T-Mobile told you)
(The Vibrant? Not so much :-P)