What kind of server specs should we go for? Any way to determine that?

We are building a Rails website that will launch soon. We are in the
process of selecting a server. Right now, we are considering Digital
Ocean as a hosting solution. But our concern is on the specs that the
server should have.
The application is simple and have basic features right now. For guests
we will provide simple and advance search on the directory which lists
Developers and Designers. Then guests will be able to send enquiries to
developers/designers. Registered users (Developers and Designers) will
be able to create a profile and portfolio with projects.
The estimated traffic on the first month will be 200K visits and about
500 signups by designers/developers.
We are not sure what we should consider regarding server’s
specifications. Are there any data or way to simulate-calculate our
Our initial thought was to go with a 4 cpus, 5TB transfer server. Yet
again, we don’t know if such server is satisfactory or not. We just
don’t want to see the server go down.
Any advice, guidance will be really helpful. Thank you.