Technically I think the only thing you need is the $30 data plan (required). But you'd probably want a voice plan (basic, select, connect, premium) and a texting plan. I believe the $30 data plan only covers email and internet. Thus, the cheapest your monthy bill can be (before taxes) to have voice, internet, and texting is $75/month (smallest voice plan = 450min/$40, data plan = $30, smallest texting plan = 250text/$5). Theres no point in going above select in voice plans since connect and premium include data usage, and that is what the required $30 data plan is.
Verizon has indicated that their $45 monthly data plan is required if you'd like to connect to a corporate Exchange server. I haven't tried that functionality so can't vouch if they may block certain ports on the $30 plan.
I have been told from people on this forum, as well as random searching on the web, that the $30 does cover exchange... I will tell you definitively tomorrow when i get my droid w/$30 data plan in the mail.