Lots of questions. I'm assuming you're looking at a 2007 International CCD 23D with the front dinette, side couch, and rear corner bed. I've been keeping an eye on the recent market for those.
You can't use NADA - it basically doesn't track with what Airstreams sell for. New trailers are commonly 15-20% off, not 25%-30% off. (If they were, I would have a new 23' Serenity in the driveway.) That said, you can buy a new one (that's light on options) for the asking price of this used one.
2007 was the first year for this model and it appears to have had a short run that year. Lowest asking price I've seen for one has been $33k with a typical price of $35k-$36k
Floorplan-wise, it is nice and open in the front - that's what attracts my wife and I to it. The front dinette turns into a bed, as does the lounge. Would I want to travel with 3 adults in it? There are enough beds, but it will be tight for that time. It would be rather nice as a solo trailer, but the bathroom is tight and there isn't much galley space.
Max weight on this trailer is 6000 lbs so the tow vehicles you mention would be fine. Note that you could tow a 25' trailer with a Touareg or F-150 easily. Yes, that is bigger and heavier, but the floorplan is quite a jump in livability.
Hope this helps.