The user cannot set the expire days during ad submission. Only the admin of which can be done through the Classipress settings including the ad packs features.
Yes, the user can renew their ads and emails are sent when their ads have expired.
• When user inserts an ad and sets it to expire in X days (what is the maximum? ) can the ad be renewed by user and can he be alerted via email ahead of time about expiring ad?
If I understand it correctly, no feature on this yet on the current version.
• Is the anonymous email feature so buyer/seller does not expose their actual emails?
Classipress is a Wordpress theme and the official Wordpress had site provided some detailed guide
http://codex.wordpress.org/Moving_WordPress on moving from one host to another and etc.
• How hard is to move the entire app in case of cha changing hosting providers?