Brad,
I did not read through everyone's comments and replies. I did see that you wrote the rig was stored indoors all it's life. A rig stored that way is well worth the extra $4000. Under the assumption everything is in good order and the floorplan meets your needs, I would "Go For It". Keep in-mind that it will likely leak rain water, if not right away, then after your next trip. Make sure you go over the rig and seal up all questionable seams and such using the right products, for the brand and model is known to leak at that age.
ADDING LATER:
After looking at the rig on CL, the seamwork appears concerning. If there is any mold or mildew smell inside, I would not buy it.