The goal is to keep the interior light, functional, comfortable and simple.
Several other recent -3 builders have installed the Oregon Aero seats with apparent great satisfaction. Currently that is the plan. A two part seat with a removable back rest to accommodate a backpack parachute when desired. While the original seat back is fitted and installed, the bottom is attached slightly forward of what may be ideal for my height, I may end up modifying or fabricating a new unit to better accommodate the parachute and my fit.
As the plane will be based in the sometimes cool Pacific Northwest, an electric seat heater is planned. Some level of insulation will probably also be desirable.
Side arm rests and some type of padded instrument panel visor should round out most of the remaining interior appointments.
Most factory build AC do not have the interior primed. I saw AC from the late 30s in the Seattle area that were not primed. Some would have light corrosion but nothing major or very few did as most problems were with fabric AC in the tail area due to water accumulation. It just really adds weight not much else and if you are in eastern WA say don’t bother too much for no gain. I did prime mine with zinc cromate. The one from CO didn’t have any and nothing corroded on it other than my temper and money I bought it for. LOL Good luck, ps was an A&P for a while in Seattle.