Older Class C's-any to stay away from?

anything of that vintage with a rubber roof I would be wary of.
bumpy

The E-450 chassis is heavier duty than the E-350. For a few years what is now the E-450 was called the E-SuperDuty. The differences mainly relate to how much weight the chassis can carry. In either case, there's often not a lot of excess capacity for you and your stuff.

The most important thing to look for in a used motorhome, particularly a fairly old one, is how well the maintenance has been kept up with. A well-maintained entry level unit will give much better service and have many fewer headaches than a poorly kept top-of-the-line unit. In particular, look for (and avoid if possible) any signs of water damage, inside or out. In your price range, it may be hard to avoid every last spec of water damage, but do be careful to understand how much there is and what has been done to fix it. Often what appears to be minor damage is rather more significant once one digs into it.

Since you want to tow, do check the specs on the hitch. Sometimes motorhome hitches have unusually limited tongue weight ratings--a hitch rated to tow 5000 pounds might only permit 350 pounds of tongue weight. That's not a concern when flat towing a car or using a car dolly, but can be a big limit for trailers.

For use with a boat, a front hitch can be a very useful addition for launching.