I just purchased a Gravely Pro turn 100 series. Its a 52in deck with the 23hp Kohler EFI. The day I got it I zipped around on it for a few minutes but didn't really get to try it out but already I'm in love with riding and the speed I can cut at is amazing compared to my hydro walk behind. I have a Gravely 36in hydro walk behind and I can't even keep up running full speed behind it. A sulky would be beneficial for small lawns that are gated but not for the bigger lawns or ones that are more open.
As far as the quality of the Gravely zero turn. Its built tougher than the lower end series. The deck has the 1/4 adjustments and is just an overall more commercial duty machine. They sell it in 48in but the way I look at it is if I can't use a 52in or it won't fit through the gate then I probably shouldn't have a zero turn on the lawn anyways.