In terms of learning new things, trusting that it's secure production ready, and bootstrapping when possible. React seems interesting, but it requires a lot more additional components as you only seem to get the view. I've used RoR before but for something like this performance could be an issue. The good thing with rails though it has basically everything needed with plenty of scaffolding. Angular is a monster and that at least would give the entire MVC but seems to be on the decline and is not a friendly framework.
I've had other posts where I'm trying to find a good CRM, and have come to the conclusion that some will get me close but ideally building a bespoke version would give me full control and be able to add whatever features I want without arbitrary limits. Additionally, since it's a larger undertaking compared to previous projects a lot of experience could come from it.
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)