Hello there!
Flexibility is abundant in Google Apps. We can connect Google Calendar, Docs, Graphs, Forms, Gmail and almost any API you have in mind.
We can create nice HTML emails for the clients, set reminders, send invoices in PDF, etc.
I have recently completed projects connecting APIs from internet data companies, music databases, internet calls records, etc.
As it comes to databases, I have some experience with developing them, but it isn't something common thing to associate with the Google Apps, unless we are talking about millions of records and billions of data points.
Let me know if you have a minute to go over the details, before we can talk about exact timing and price.
Cheers,
Denis