For a used RV, I think the most important thing to look for is its condition—how well the previous owner maintained things. A lesser motorhome that has no damage is more desirable than a top-of-the-line one that's leaky and has water soaking through the ceiling or walls. For class C motorhomes, the cabover section is probably the most likely to be troublesome for leaks.
The appliances and many of the fixtures are very similar among brands and trim levels, at least in the lower to upper-mid level RVs. At the very high end, of course, the sky's the limit.
Probably at least 90% of the class C motorhomes that meet your basic requirements (length and towing capacity) will be built on a Ford E-450 chassis, and most of the rest will be on the equivalent Chevy chassis. Either one is good and reliable. With these chassises, you're pretty much limited to gas engines—the Ford V10 or the Chevy V8. Among the class A motorhomes, if you don't get a diesel pusher (which would be rather rare in 30-32' lengths, I believe), you most likely will get something built on the Ford motorhome chassis.
My personal preference runs towards avoiding slides; many others, perhaps most, don't share that preference. Especially on older motorhomes, slides seem to me to be another thing that can break and/or leak. They add weight and reduce structural strength. They do have only one advantage, although to be fair it's a significant one: they greatly increase the spaciousness inside when camping. For one person (and two dogs), that may not really be a great need.
Do pay attention to the age of the tires. Most motorhome/RV tires need replacement based on age rather than tread life. Anything around 7 years old is about due for replacement or at least thorough inspection, and much over that definitely needs replacement before going anywhere. Check the DOT date codes stamped on the sidewall. If you do need a new set of tires, it'll cost around $1000 (for a class C, more for some class A's).
If you think you'll be camping in freezing weather, look for enclosed (heated) holding tank areas and plumbing.
