1. The App Engine Standard Environment only supports Java 7. You need to use what used to be called "App Engine Managed VM", or now App Engine Flexible Environment. It is in Beta, complete with a not-for-production-use and possible-breaking-changes warning. However, I think people have been using it for a while, and they may be able to weigh in with stability of the APIs.
2. Cloud Platform can handle any kind of application. Whether it is the best choice is very subjective. I could see it doing very well with a web-based ERP or very, very poorly, depending on how the ERP is coded. However, large reports and background tasks may be better handled by Compute Engine instead of one of the AppEngine environments. It all depends on how you can break up tasks and how the pricing will fall out.