For a used motorhome, the most important thing to look for (besides whether the layout etc. would work for you) is its condition--more important than brand name, age to a large extent, etc. More specifically, look for freedom from water damage (or, at the very least, any water damage has been properly and thoroughly repaired); for appliances and systems that are demonstrated to work properly; for evidence of proper preventative maintenance; and, of course, for overall signs of good care and not being trashed.
Mileage in and of itself is not all that important for a motorhome, in my opinion. The E450 chassis is quite capable of going for several hundred thousand miles, and in nearly all motorhomes the house part will wear out long before the chassis.
Finding rear twin beds or bunks will be hard.. They do exist, but are quite rare relatively speaking.
For towing the four wheelers, pay close attention to the hitch ratings, particularly the tongue weight rating, and the rear axle payload capacity. Many class C's are limited in the tongue weight they can carry due to the long overhang behind the rear axle, if not by the hitch then by the available payload. Bear in mind that the tongue weight will cause weight shifting from the front to the back, so the actual extra payload on the rear axle will be something like half again more than the tongue weight. Once you add water and people and people's stuff the payload can disappear pretty quickly.
I'd also suggest budgeting some money for repairs and upgrades/upkeep. A set of six decent but not top-of-the-line tires will run you $1000 or so. Shocks, brakes, and myriad other things can and do wear out on older vehicles. It is unwise to stretch your purchase budget so much that you can't afford these other things if needed, and chances are fairly good that something will be needed.
