My hydraulics theory and application knowledge is getting really old. However, I think the key here is the ability to do two things at once. Your planter pto pump only has to do one thing, and the pump is sized correctly to the application (I presume).

It's not the same as trying to run a fan at constant speed and also try to provide raise/lower and do all this without causing heat issues. To do that on a fixed displacement hydraulic system with the typical open center scv's isn't easy (I presume you've found that it isn't easy even on a pfc system since you're going to a pto pump on that tractor!) That's why I was saying you would need to use a "power beyond" method bypassing the valve stack for the fan motor.

I truly was expecting to find a method in Deere's service information (it could be there and I missed it) to cobble up that planter fan motor to that specific tractor and have it work for both lift and fan speed. I've done it before with the method I described. Maybe I was being too cautious. I'd want to have someone with strong practical experience on a simpler method advising me if I were to try anything else. I still stand by my advice to use a pto pump in this situation.

edit: ordinarily I would have left this topic alone because I don't know enough about the specific tractor in question. However, the OP specifically said he was going to go and spend some money that I felt wasn't going to be the complete answer. My main intent was to highlite that more research would be needed to be sure of what would work on that tractor.