We have a DP and it is 34.5' actual and the wheelbase is only 208". It also has a 55 degree wheel cut which makes it turn very sharp. I had a 31' but was actually 33.5' long front engine gas RV with a 45 degree wheel cut and a 228" wheelbase. It had a very large turning radius and a massive tail swing. On a gas RV I would opt for the longer wheelbase even though the turning radius is so poor just to get the stability required. On a DP with air ride it isn't so much of a problem.
When i bought my coach, every manufacturer was putting their longest coaches 36 to 38 ft on 22,000 lbs chassis with the longest WB being 228 inches. One of the reasons I went where I did was the 24,000 chassis and 254 inch WB. Some one here has a formula for length to WB ratio. You might try using that. I think a 34 ft coach on a 228 WB is OK but not longer
New or used? Every manufacturer has WB specs on their site for every model. Is there a particular floor plan you want? A long WB is nice but worthless if it's sitting under the wrong floor plan. In terms of priority, WB length would be much further down my list. A consideration sure, but not the primary one.