I wouldn't go more than 3000 imperial gallons on that tractor... it's a fairly light one and you have that one hill.  As for brakes... I wouldn't even ask the question, it's an obvious yes.  I don't even know why these things are allowed on roads without brakes.  
Only thing is 1.1 million gallons twice a year is a lot of manure for a 3000 gal tanker... you'll spend a lot of time doing it.  We have a 3750 Houle on a MF 7485 (135 hp) and empty a 650000 gal Slurrystore twice a year.