I think a medium-size C would be great, especially a 27-28' with a long wheelbase. This size can be tricky because they also come with very short wheelbases. Besides riding better, the longer wheelbase will reduce tail-swing, which might matter if you venture into town much.
I would avoid large slides. My mega-slide (14' x 42") has be problematic since day one. Mine's a Schwintek, so the worst of the worst, but more important is that if it gets stuck, it's too heavy to man-handle. A smaller slide, like only a couch or a dinette, could be fully disconnected and moved manually if need be.
I would not take a Class A very far south of the border. #1 reason being windshields. You're likely to get chips, and bad roads will twist up an A enough that any chip will likely become large cracks, and you probably won't be able to get replacements down there. E-series are still common enough in Mexico and Central America that you shouldn't have trouble getting glass, and a C isn't as likely to need it in the first place. Just like here, a cracked windshield is an easy excuse for an officer to pull you over, except down there, you'll need to present your ID with a wad of cash...
I think I'd seriously consider a former Cruise America Majestic 28-footer. They are built-out super simple, have lots of payload, and can be bought for cheap. I'd have zero reservations with a high-milage chassis, and the normal RV problems (leaks, appliances that don't work etc) are just as likely with a new unit, maybe more so, because at least Cruise America takes the time to fix those problems before sending them on the road.