1- $350 / month is the cost for a very very good server. You don't need that much. Not from the start at least. You can start with just $10 a month for a decent server.
Then you can expend to better, but by then, you should already be profitable.
2- 5 developers for 1 month are less productive than 1 for 5 months. Keep that in mind. I would still recommand to hire at least 2 developers so that they can criticize each other/help each other out.
3- None of what you give is a one-time service. SSL Certificates are free (look at Let's encrypt for example). Moreover, even if you paid for it, it would probably not be a one-time fee since you  need to renew it from time to time. DDOS protection is not a one-time fee service neither, usually it comes at an extra price with the dedicated server. Finally, if you want to do security audits, it should be done at every major code change otherwise better do none.

I don't mean to be mean to your project. If I took the time to write this, it's only because I think it's for the better for you (really).
Good luck.