Depending upon how old your children are this RV should serve you well for the next 3-5 years. Trying to find a quad bunk house would seem a bit difficult. I'm not in the market for one, but have always kept an eye on used RV's online and only remember seeing one in about the last year or so. If your kids are old enough they sometimes prefer to sleep in a small tent outside the camper. Mine did anyway, but they had to because we started with just a 20' TT and we had 3 kids in their early teens.
You certainly have enough tow vehicle for that size/weight TT. I believe that size TT would have a 6 gallon water heater and should fully heat the water within 30-45 minutes at most.
The seller sounds like a good person to buy from. He probably keep everything fixed. You might ask to see his repair receipts. I do that for any used vehicle. Good owners keep things like that. I also wouldn't worry about the skylight repair. If a dealer did it it should be ok. But having the receipt just in case of a future leak would be helpful. I stepped through my skylight last year and had it fixed by the dealer. No problems since.