Your third function should have the same flow as the rest of your tractor.
I would suggest a third function vs a diverter valve. More flexible.
I don't know the specs for your M, but it definitely has enough to run one.
I have the Land Pride SA10 on my BX. I believe it needs 4 - 8 GPM, my BX supposedly does 6 GPM.
I was told by the salesman the SA10 was designed for at least up to the LX models.
The larger Land Pride SA20 needs 10 - 30 GPM, which may be more in line with what you have.
If you can, go with the larger one. More power and options available.
At least for the SA10 you can either mount it on it's own quick attach frame or use the LP pallet fork frame.
Not sure if that also applies to the SA20. Saves a few dollars if you can.