Hello,
i can build a prototype in unity, as yr budget is low right now, here what i can do for u for 100$, i will build a game where the player u defend a point a ganist some monster, it will be other player AI , where u can give them some weapons and they will help u.
as for milestone is a system on freelancer where u put the money on the site once the job is finish u release it for coder.