Hi Gary4gar, Read the points that you stated in your question. If you focus on growth (clearly your wins are going to bring more data updates as your increase the customer base) you should look into a solution that does not tie you down with data model and also does not expect you to worry about sharding in the future. Having done sharding and when one keeps growing the data volume and having to do shards it becomes painful and hurts the development flow. Been there many time and done it quite a bit myself. (Am a technical guys myself, http://www.linkedin.com/in/sukantag)

Look for a NoSQL solution which will help in the flexi-schema model. Helps you add different types of data models relatively easily. Check out Aerospike (http://www.aerospike.com), the product scales like scaling problems have disappeared from this planet. All of the characteristics that you would need based on what you have stated is in the product. Best of all it is free for startups. Just give it shot. Open Source server and client libraries in multiple programming languages. (Am a big "C" and Python guys so like them client libs a lot). Lots of startups and many large software companies use them.

If you like it and works for your application tell others, if not tell us what did not work.

I ain't sellin and you ain't buyin;

Ciao