I am a fairly new C owner and have just about completed a cabover rebuild, so I am just chock full of opinions. ;)
The class A will be more likely to have a "real" bed in back, but you can find a nice class C with a queen in back. As to the driver seat, if you really have comfort issue then you could purchase an upgraded driver seat with all sorts of adjustments and even air suspension like a big rig seat.
Mileage on a class C seems to sit anywhere from lows of 6 mpg up to highs of 11 or 12 mpg. If you can find a diesel, they will get better mileage and have more torque. They will of course cost more. If you buy a more recent production Ford based RV, it will have a v-10 engine. Those seem to be pretty reliable and more powerful than the older v-8’s.
I didn’t think about the shade aspect, but I totally get how sitting in the sun, behind a giant glass screen all day could be rough. I got used to the overhang within a few miles of driving my rig and sun was never an issue. What can be an issue is engine heat coming through the doghouse or floor. Be mindful of that when test driving. Take it for a good 15-20 minute drive and you will be able to feel if that heat soaks its way in.
I really like our floor plan. It has a 3 person couch on the driver side and two captain chairs on the passenger side. There is also a narrow pop-up table between the captain chairs. You can pull the dining table up out of the floor (two posts) and stow it. That leaves the floor clear and it’s much easier to walk around. It also allows the couch to fold out into a bed that can fit taller people. The dinette style center beds are no good for 6’ or taller.
Slideouts would be nice, but I read a lot about leaks, chafed wires and other little niggles. If I were full timing maybe it would be worth the tradeoff, but I like not having slideouts to worry about.
I TOTALLY AGREE about water damage. You sound like you want something ready to use and do not have interest in working on it. If that is the case, then water damage is a for sure deal killer.
Hope you find the perfect fit.
Jose