28' class A's are practically non-existent.
In my opinion, there is less difference between a lower-end (gas) class A and a class C than there is between a diesel pusher class A with air suspension and a gas class A. The main practical difference between the C and the A is the shape and configuration of the cab area; a class A will have a bigger windshield, and you'll be closer to it and higher up, and not have a cabover "cap bill" sort of thing to shade it--which is sometimes nice and sometimes not so nice.
I would suggest looking at a bunch of units and seeing what you like. I'd also suggest thinking about renting a motorhome for a weekend or week, as it will give you some better idea of some things to look for or avoid that may not be immediately obvious at first. Some motorhomes have rather puzzling details, like no good space to put a trash can or having a shower and sleeping for up to six people but only one little towel bar (and perhaps no good place to add another). Many have very limited kitchen counter space, which may not matter much to you or may be a rather important consideration, depending on how much you cook. Some have inconveniently located television sets (or more TVs than a small sports bar). And the list goes on....
Buying used is a good plan, in my opinion. Be more concerned about the quality of the maintenance of the unit than the quality of the initial manufacturing, though of course both are important. A poorly maintained motorhome, particularly with water damage, is a poor purchase.