I haven't been around the T6 transmission but the way I understand it is a newer technology that matches the pump with the motors while giving better cooling thus longer life plus extended maintenance cycles.
This is how it is advertised:
T6 hydrostatic transmission with tandem 16cc Parker Hannifin pumps-in-reservoir, in-line CoolFan for cooling efficiency and long service intervals, design-matched Parker high-torque wheel motors.
I am with you on the hours, could be expensive if regular service was not followed. It would appear that the number of hours vs. the age of the unit would suggest a lawn service or municipal that owned it most likely ran it 30 to 40 hours per week during the season?
Are there any service records or do you know who was the previous owner? Does it look well maintained? In my opinion you can tell a lot about a unit by its appearance. Also I would talk to a reputable dealer and find out the track record of the T6 transmission in regard to hours.
It is a great price, would be hard to turn down!
Good Luck