The only time you'll get email to repopulate from the BES is when you activate a new device. It will populate as per how the Admins have it setup. If you wipe the device and reactivate the same device you'll get no emails. If you activate a new device you'll get a certain amount of emails. If having messages on the device is important then it might be wise to make frequent backups using desktop manager.
As far as memory goes, I would be carefull about anything less than 15Mb. Just do a battery pull once a week and you should be fine.