You will have a hard time finding a rear bedroom in a class C that short; there's not enough length behind the wheels for the bed. You'll be more likely to find a corner bed. In either case, there are very often things in the way underneath the bed, either water tanks or storage compartments or other RV systems.
I think you'll have an easier time looking for a dinette and/or couch area that can be made into an office area. There still may be things underneath the dinette or the couch (like compartments or systems), but I think typically they're a little easier to deal with and you'll be more likely to have a decent size open space in a relatively short RV.
As far as brands, for used ones--particularly ones that old--the condition is in my opinion of more importance than the brand. The chassis is almost certainly going to be a Ford or possibly Chevy van chassis, and those are pretty reliable and well-understood. The biggest things to look out for are evidence of leaks or water damage (which can be quite a job to repair and is generally worse than it first appears) and proof that all the systems work--at least the ones you care about. That means seeing the generator run, the furnace work, the water heater heat water, the stove and oven do their thing, the fridge being cold, etc.
