Hi, actually we have some web apps in amazon aws and we know the infrastructure.
We have some doubts about this project. We don't know if you need the architecture definition and software process, or you need to implement the solution also.
The actual price for this proposal is for both, architecture and implementation. In other case, please get in touch and we rearrange the proposal.
To develop a good architecture design we'd need to know some data, like estimated number of transactions, connections, process, software of application to be supported, and so on.
Sans all of this data our first attempt of architecture could be a operative system linux based like CentOS or Ubuntu Server and to provide a bunch of well tested open software tools to administrate the system to reduce the initial cost.
About the AMI size, it is necessary to know the previous requirements, to better resize the memory and CPU, and the software platform also, (java based, php, python, etc.).
For example, a common web architecture could be Ubuntu Server with EBS +apache+php5+mysql, a java based web architecture could be jdk6+tomcat6+mysql, (if need EJB support we change tomcat by Glassfish), or a mixed architecture: apache+php5+tomcat+mysql.
As we say, all depends of requirements.
I hope can help.
Best Regards.