How should I approach my EU2 emergency plan?

I looked at other SO questions regarding EC2 backups and it provided me with a good foundation, but I'm still a little confused about how I should approach my solution and develop a contingency plan. Most of the questions are quite specific, but I have a pretty vanilla installation, and I think this information will be useful for future users. Let me provide my basic setup:

• The main small instance
• Clicking files on S3
• Launch MongoDB
• Nginx launch

Now, due to the ephemerality of EC2, it is obvious that I need to bind my EC2 instance to EBS to ensure persistent storage. The reason I'm trying to develop a contingency plan is because I am worried that my instance may disappear at any time (due to crashes, etc.). If my instance disappears, I am concerned that I will have to deploy a new instance and reinstall all my applications before restarting again. A few questions:

• How do I back up my instance to make sure that if it disappears, I can quickly restore it (preferably without reinstalling all the previous software)? I do not need a series of backups, only to previous (or weekly) backups to guarantee the existence of a previous working version that can be launched quickly.
• If I use EBS instead of instance storage, does it essentially act instead of my hard drive? So, if I have MongoDB installed, I assume that the database is being written to a cab located on EBS?
• If I go with a small copy with 160 GB of storage and I use EBS, will I need to allocate 160 GB of EBS from the shutter or just for 160 GB storage?

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