I'm on my first Ventrac... a 4500Y with a 25-hp naturally aspirated, liquid-cooled Kubota diesel engine. I just hit 200 hours. I recently had a bit of stalling, which I resolved by changing the first fuel filter (the one closer to the tank). The filter had debris of various sorts in it... apparently leaf dust, and some small winged insects.
Other than that, all that I have had to do has been routine maintenance... oil and oil-filter changes per the schedule in the Owner's manual, air-filter changed per the schedule in the Owner's manual, and greasing of the various zerks.
I have no regrets about my purchase.