Backend Development language for startup
I'm launching a startup that, from a programming perspective, will be similar to Uber (lots of Google maps interaction, hopefully a lot of users, real-time info being regularly updated, a lot of real-time interaction, gps updating, mobile-heavy, etc). I am debating what language this should be coded in (and will be working with an ***** developer once I decide). I would really appreciate some advice on whether to focus on C#, PHP, or Java for backend development work. (And, down the line, would I likely want to stick with one of those, or would something else be preferred?). I'm unsure how important performance is, but would expect that my initial version would need to have 15,000 users sign up to get enough funding to hire a really sophisticated development team to revamp the whole business.
Anyway, advice on which to pick for my startup is appreciated.
One note: One of my potential developers (the guy who I like the best for building my alpha product, but someone who does not seem to have the expertise that some of the people do on this forum) is quoting me only a 10% markup to code in C# or Java, after obtaining the same deliverables. Tangential newbie question: Will I need to additionally pay for licenses or software if we choose C#? (I'd like to be able to create and test the code myself, too, even though I'm not the one developing it). If so, how much should I expect to pay out of pocket?
(The main issue is that this guy is going to probably put together an alpha product, and hopefully it's good enough to keep the code and have someone else do a beta product and scale up ... but I don't really trust him to know which is the best language as it scales)
As a side consideration: I'm slowly learning to develop myself, so the ability to understand the code or pick it up quickly is preferred.