Probably the worst thing you can do is start with some complex clustered architectural design.
Just start on a single server with technologies that are scalable, and design with future scalability in mind. Also design in the ability to capture detailed performance metrics of every tier. When, and if your application usage grows, scale the parts of it that need scaling.
The biggest issue with scaling is usually the database, and for applications where you are just using the database as a simple persistence store for user settings and simple small data sets, you are probably best to go with one of the many scalable "NoSQL" type solutions such as MongoDB, as they've got scalability baked in for free. If you're trying to run heavy duty analytics that join and aggregate massive datasets, there are single DB clustering solutions, but they aren't cheap. You can always scale out SQL databases horizontally, but then you've got issues cloning and replicating, though there are a lot of products in that space, both free and commercial. A cheap place to start would be with PostgreSQL, which appears to have multiple open source replication products.
I don't think there is anything inherently limiting to sticking with Java. It's what you know, and the toolsets are deep and rich. No, it's not the hot new thing, but sometimes that can be a good thing.