If it was me I would try and stay below 500 hrs, not that more would be any problem. Condition is a big thing too, see how clean it is, check for oil leaks, belts, blades all add up to money too if you have to replace them.
I think on the older GT235s -99 through 2002/3 or so, the hour meter was option so many or most of those will not have meters. You will need to judge by condition, how/where it was used (property size, mowing only or with snow remova, etc.). For the newer units with meters, machinefinder right now has a fair number of GT235s with the highest hour unit at about 830 with plenty well below that.
I was in your shoes in 2009 looking for a used GT225 or 235. I looked at a number of units and didn't have to consider anything over 300. There were plenty of well maintained, excellent condition options with low hours so why bother with the high hour units. There isn't much of a price break of something south of say less than 1000 in my experience, so why buy something with 600 when you get one with 250? And 1500 hours is still 1500 hours, not matter how well maintained and if it is a pristine unit, my guess the seller will still be looking for a premium price.
The GT235s seem to have been fairly popular, so there should be a fair number out there. I'd look for something under 300 hours in very condition personally, maybe less than 400 if necessary. You want to buy from someone who overbought, say a GT235 for 1/2 or 3/4 acre, no snow removal, no ground engaging work, just mowing and did your routine maintenance.