Front end: We use Ember because of its balanced focus on progress and stability. The team behind it is solid and batteries are included.
Application: Ruby/Rails, Elixir/Phoenix. Rails is great for getting up and running quickly, and your problems with #scale won't be on the application later. Elixir (really Erlang's BEAM) is great at quickly spinning up/down lightweight processes that handle raw data. So if you want to build a CRM and, say, manage emails, then it could be a good fit. Or if you want to process a 1,000,000 row CSV import, etc.
Database: Postgres. Binary JSON columns give you flexibility, plus PostGIS for all of your geospatial needs. Structure as much as possible to take advantage of the smart people who have spent countless hours building a world class database. You'll appreciate their focus on correctness.
My overarching advice: push everything as far down the stack as possible. If you can do it in Postgres, do so. Data correctness is your life; better that your DB throws a fit than your application serving bogus data.
Happy to answer any specific questions!
(And I wouldn't be a YC CEO if I didn't take the opportunity to mention that we're hiring! http://seneca.systems/careers)