For me, and my skills aren't even close to the level of a lot of you guys, Plug wires are a must ( and if there is not kit coil I add that too) and the upper radiator hose since they are really the most obvious items that are seen. And since the majority of models don't seem to have alternator or generator brackets I recently bought photo-etch brackets for those.
My models are not meant to be show pieces and are displayed in as enclosed case with the hoods shut (and probably never to be opened anyway) so exterior finish quality and details are mostly important to me. Just me 2 cents worth fwiw?