i bought a used Gravely PRO-50 for $900 4 years ago. It paid for itself in no time. If i did it again, I'd look for a used Hydro machine, my machine doesn't drive very well when its wet, so i avoid cutting when its wet when possible.
With all the time you'll save by using a commercial deck you'll have more time to take on more clients, or relax.
Efficiency = Profit