Before you go into all the nice things to have fitted look into the possibility of getting an airframe that can accept an auxillary tank. We discovered that as our airframe was not modded for an auxillary tank, having one fitted would have meant major airframe work. 2hours 20 minutes endurance is a real hindrance for long trips. You can spend as much time on the ground refuelling as flying and it costs you landing/handling fees every time you stop. We also went for low skids as more convenient for embarking/disembarking. Small boarding lights that illuminate the skids and steps are really good. Weather radar we did not consider an advantage nor did we go for air con. Tinted sun shields and windscreen wipers are a must. As performance is a problem we fly usually with only two rear facing seats fitted and a convenient cover is fitted over the third seat fittings. No bar or other weighty fittings.
I have not included here the piloty things as the list is very long.
Try and keep your ship as light as possible. Ours weighs in at 1832.