1) The only difference between the OC plans and normal ones is that they have no monthly billing plan. Pay by the year or not at all
2) Yup, on all the plans.
3) Yes, webmail is enabled on all plans.
http://yourdomain.tld/webmail or
http://yourdomain.tld:2095
4) No, you get a pro-rated refund for cancelling early.
4 #2) You set the email size limit for each account. Can be as large or small as you want (within you plan limits, or course)
5) There used to be some trouble with AOL's filtering, but that was caused by people using the "Email Forwarding" option and was resolved after Surpass got all new IPs for every server.
6) If you used the current servers as mail.yourdomain.tld then there will be very very minimal downtime during the switch. Because it relies on the domain, when the domain switches servers, so would the client.
7) I would personally use Surpass's servers since you're paying for them, but it's entirely up to you.