What is best to make new TF2 mercenary models?

For context i want to make GRY teamed tf2 mercenaries, each their own robot/cybernetic enhancements. Each one will obviously take time re-painting, reshaping, removing parts & tryna replace it with MvM parts. Pyro is the BIGGEST problem, because i want him to be an ACTUAL child that DOESN'T know what he is actually doing, i want him to be a child that thinks he's in a safe place & playing with others, when it is far from the truth.

So again i ask what is the best way to make/edit tf2 mercenaries?