27 feet is at the upper limit of RVs fitting into state & national parks; some of those CGs will still have sites too small. That said, the size is a good compromise between interior space you'll need and fitting into campsites where you want to go.
Options: air conditioning, absolutely! You may find you want an awning, but could start without it. Microwave can sure be handy, and TV, since you have young children. (Not to encourage them to spend time inside instead of enjoying the outdoors, but there will probably be times - in bad weather, for instance - when a TV could come in handy, especially with a DVD player that works with it.) Extra/large batteries (we have 2 6-volts instead of the standard single 12-volt) can be very handy for "dry camping" (i.e. without hookups), which is what you find in most national parks and a lot of state parks.
No experience with Airstreams, but when we visited their factory, we were sure impressed with how they are built. Hope this one works out really well for you! If not, resale value should - I would guess - be better for an Airstream than for virtually any other brand.
