Well, the best backup policy would be to do the backup off-site or off the server as they would ensure its integrity if you need to recover. As to how often, it depends on how dynamic your datas are. For the norm, once a day to once a week should do fine.
As to whether you need managed service depends on how competent you are at server management. If you know the OS inside out, know how to do updates, secure your server, troubleshoot and have the time to do those then an unmanaged hosting should be fine. If you lack once of the few skills then you find that managed plan would fit your needs. You could always still get the ev1 server and get someone to manage it for you.
Try to do it at odd times of the day too. Such as like 3 AM EST. Backups especially on larger HD's can really bring a server to its knees depending on how well you back it up. I've taken a dual xeon to loads of 7+ from just backing up one 80 GB HD.