25k should provide you with a decent class c. Class C's are great because they have ample sleeping arrangements. We had a Jamboree that did just fine for us. It was a 27 footer and had the over cab sleeping area dinette bed and double bed in the back. We used that on many trips with our six kids. We used it on beach trips, travel trips, camping trips and skiing trips so I think you have a great idea. I think it is the perfect RV for families with children.
Check the hitch that your future rig has for weight rating.
Get one with the largest holding tanks possible. I think that is one of the more important items. On board generator is nice but a portable will be quieter so either way will work. With a portable if staying for a extended period of time it is nice and quiet as you can set it away from the rig. Of course with a portable you want quiet as possible.
Check the tire age on the rigs you are looking at. If more than 2 ot 3 years old plan on having to replace them pretty shortly. 5 or 6 years are about the end of the safe lifetime for rv tires. They dry out and blowout. Ask me how I know:)
JMHO