1) In my opinion, no to the extended warranty. Any major issues should show themselves during the factory warranty period. There will be some issues so be prepared for that. That's my opinion and others may feel different.
2) We have a Cherokee 32B, measures 34'-10" tip to tail. We have no regrets on the length. We do alot of state and national parks and always find a spot. Electric sites regardless of length are always a low percentage item in our MD state parks. Typically (1) loop out of (4) or more will be electric. For that reason we always reserve our sites early (we normally have all our trips booked for the year by the end of February). This is required around here whether you have a 25' or 35' camper if you want electric. I would not want any smaller, we really like the boys having they're own bedroom with a door.
3) Your price seems like a good deal on a trailer that should serve you well until the kids are too old to want to camp with you any more. Post pictures when you get it! If buying new, I would want the 15k A/C upgrade instead of the standard 13.5k A/C. Its less than a $250 add-on.