Yes you do but still cheap depending on what you have in mind and it’s not an initial cost out of your pocket but a cost that can be deferred up to the day you release your game. Kind of like try before you buy as you are covered under the personal use license up till then.
If you want to use Genesis, which is state of the art, then the first purchase you will have to make is the body and head morph sets for the male and female Genesis base that will allow you to generate parameter driven characters. With the morph set you can create “any” character you want be it male, female, child, zombie, monsters, all off the same base that you can us as an injector in UE4 that will allow you to create thousands of unique characters during run time.
At its core what best describes Genesis is this.
You would still have to code the AI logic but the magic is Genesis is a “true” parametric character editor with a memory foot print equal to whats necessary to make a single channel pipeline functional.
Risking sounding like an ad this is how Assassins Creed can have 5000 NPC’s in one area with out bloating.
No disrespect but Mixamo can’t do that…yet.
Overall though I would say that a vending machine license is a good option as to buying what you need when you need it and not because you have to decided up front what best fits your need.
Just saying. ;D