I believe he was the mastermind and that he was prepared... He was prepared for an abduction and a sexual assault, he wasn't prepared to murder a child.
When TLM came back with an older child he made the decision that she would have to be killed. Maybe a child of 4 or 5 would be easier (please excuse my bluntness) to rape and release somewhere and they not be caught, but a child Tori's age can describe them, give details about the car, tell exactly what happened.
I think too that when Tori was brought into the car she was asked her name, and maybe she spoke of her parents, I think when TLM made the connection of knowing Tara she realized too little Tori would have to be killed, that this wasn't just going to be a sick sexual fantasy brought to life... This was huge and there was no turning back now.
MR had his funds for drugs/gas no extra for murder weapons because he sure wasn't going to use anything of his own.
This is all just my opinion.