Hi, hope you are doing well. Let me drop a few words about myself first. I'm systems and network engineer, currently employed by software shop (AWS + Java + PostgreSQL) with tons of experience on AWS in particular.
In regards of requirements you outlined for the project. Its rather simple to get this job done. To make it simple, I can write puppet modules to accomplish configuration of application server. Pretty much same in regards of DB backend. I'm not quite sure about using rds in your case - it is managed by AWS, yes, but pricing is too high. Any way, its your call. The next question is - what about backups? Is it a part of a plan? Than, what about the load? As you mentioned, the min requiremrnt for RAM is 512 MB, but what about CPU, disk I/O? I think it would be beneficial to catch on skype (just send an invite to ven1meux on skype) and have a chat to clear out all requirements and than get an agreement on schedule and pricing.
Hope to hear from you soon,
Regards,
Taras