I've heard these scripts can bring a server to it's knees if you're not careful. What features should you look for in a web host to run an Amazon AWS website?

Never heard that before!
Im pretty sure most servers can handle this script,as long as you arent pulling in the whole amazon catalogue.
Stick to a few product categories and you will be fine.

Any script can bring a server to a crawl if not implemented correctly. Just be carefull and be aware of what the script is doing.
If you think it might be a dog, then let your webhost know ahead of time that you are using the script and you're not sure what kind of impact it will have on the server. Hosts don't like suprises when it comes to their servers.