To get enough money to do what I need to do, is it better to spend money on unusual fish eggs, environment upgrade, a 2nd tank, or what? I am trying to figure out how to make greenfin spotanus & some others..so which one of the things I just mentioned would I start with to be able to get the others?

I always start with the common eggs, and breed the basic fish to save money for the second tank which gives more space for breeding and more money.

And the greenfin spotanus is easiest obtained in the common eggs.