There are several third parties that say they have a working MMO style network infrastructure set up and ready to go. They each cost like 1000 bucks per license (which is cheap for development studio, but not so cheap for me and my buddy fucking around). Our work around was to do this:
While we are prototyping the game, we essentially bifurcate the script on all objects into what we call "smart code" and "dumb code" using Unity's default networking "MyNetwork" testing. Said differently, we have an if statement on all relevent scripts that says, in essence, If I'm the player, use the smart code, if I'm not the player use the dumb code." This approach is not efficient at all, and is totally a "hack", but it allows us to effectively prototype for a MMO without developing the full server infrastructure on a budget.
The idea is that, once we are willing to spend some cash on the project, we will be able to, with some effort mind you, move the "smart code" to the server and the "dumb code" to the client. Also, we set up separate script to serve as the "database" where all it does is set variables. Also inefficient, but saves us the hassle of managing a DB with a static address. We can just each mess with the full code base, all within Unity at all times. We use git as version control, and we haven't had any issues at all with it.
And, we are just a couple of guys that thought "all mmos suck right now, lets make one." I am sure that a team that knows wtf they are doing could prototype combat for a few months in a way that doesn't screw them/force a complete re-write when they go full MMO.