A lot is going to hinge on what condition it's in and was it well cared for by its previous owner. I looked at all kinds of used rigs spanning several years and that answer will be all over the map depending on what you're looking at. There's some good pre-owned stuff out there, but you really need to do your homework to find the good ones.
Floor plan is a huge factor too, so make sure you like it both closed for travel (make sure you can access everything!) and also opened up for your trips.
In diesel vs gas - it really, truly, honestly is subjective. There's no right or wrong, but rather what will work best for your own particular needs. How the heck do you figure that out, right? Well, first and foremost, see if you can get some test rides in a few diesel and gas rigs too - that can answer a lot of questions for sure right out the gate. The diesels have better rides overall, but newer gas rigs are not the horror stories that many make them out to be.
I asked many of these same questions too, but ultimately found I had to physically get out there, get in each type, and generally 'feel' all these things out for myself before I could pull the trigger on gas vs diesel. And, since you're looking at later model used, this is even more important to get the overall picture of the one under consideration. After you do your internet and forum reading, the only way to answer that question is to try to get some real-world experience to get closer to what your decision will be.
As you can see from my signature, I went with new gas, since that is what worked better for us (primarily vacation use). Our rig is 37'7" and has a longer wheel base of 242", which I think helps the ride tremendously. From all the reading I did, it sounded like a lot of folks thought their gas rigs rode terribly, but that hasn't been our experience (thank goodness!). But, the only way I could "know" that, was to get out and experience it. It's very likely that someone sitting next to me might not think the same!
Not sure I'm that much help here, but best of luck in whatever you decide!