Heavy EC2 user here. This doesn't solve your problems, if you want to do this right, setup an EC2 Auto Scaling group and build an image each time you need to change your server. That is the proven way most large deployments work, including Netflix.
If you're only scaling up a few instances at a time then your number one consideration is probably making life easy for yourself.
edit: grammar
There is also a video of a talk about it: https://skillsmatter.com/skillscasts/6057-herding-cattle-wit...
We based our approach on Netflix but ended up building our own tools which we've now open sourced.