You are not limited to a length as much as weight. I have seen some HUGE units on short beds, honestly I would not want one as they do not look safe.
My TC is a 92 and has everything that you mention without the air (around here the air is not a big issue and I did not want the extra weight of a air unit that I would seldom use), and I do not have an outside shower but for me not a big issue. I do have a long bed as I had the option of buying a truck for the TC so that is something that I decided on, again not a big issue as I see many short beds with TCer's.
The biggest issue for age is to make sure that they are solid. If they have no water damage then age is not a big thing. Water damage will kill a new unit as much as an old unit. Again for me - I went with a Northen Lite as I wanted a lighter unit and I love the fiberglass for being water proof and better in the cold.
After a recent 1800 mile, 9 day camping excursion up and down the oregon/norcal coast, we want a truck camper more than ever.
I am definitely thinking the fiberglass type, also. I like the weight savings and insulation, and I think they look much cooler.
Also, I think I will stay with 8.5', if only for maneuverability. Are there many of these with a wet bath? Brand suggestions?