I think terminal buildings as a bare minimum should be a priority to add to the realism of the game. These could be visible from a certain distance or turned off in the settings to reduce RAM usage for different devices etc.
As much as I love IF, taxi'ing around international airports without taxiway lights and terminal buildings just doesn't cut it (as a paying customer). Even low quality polygons would be better than painted outlines?
As for time and resources - compared to the detail of the aircraft I don't think basic terminal buildings would take too long (speaking from 3D modelling experience). Maybe get the community to submit 3D models of buildings if resources are that restricted (a bit like the google maps 3D buildings project).