Depends on what customer you want to target. 65 hp will run an 8' and if you get a med to heavy (preferred) duty with dual wheels, 2ea 4' cutter blades you can cover good ground rapidly and have a clean cutter. That size will work in a 3 pt nicely and would be my preference since you will be mobile. 8' will travel the roads on it's own or trailered with no width permit problems. I had a Mohawk as I described and more than enough power with a 70 hp JD.
If you have small jobs with a lot of ground variations, you want a small mower or you will have scalping problems. You might look at a conventional riding mower for that sized job.
On my tractor work, being retired, I figure $10-10-10.....me-equipment-fuel per hour and if I am not working at the job, I deduct the dead time. I see signs $40/acre for commercial guys, no mention of travel and all so I don't how they handle that. You have to figure in travel time too. For nearby jobs it's in the pricing. On long ones, on the first ones do it for free and get an idea as to your costs to get to and from the work site and price accordingly. Nothing wrong with pricing the job from your farm and back time, just put yourself in the customer's shoes and treat them accordingly.
