Now come on IO you know better than to get into a GPS debate with me.
The question was about airways and GPS is specifically permitted as a PRIMARY nav source enroute. Using GPS elsewhere is not specifically permitted or denied it just makes good sense!
Anyway back to the point, it is not just appraoches that require an ADF take a look at an airways chart and there are still a lot of NDB's that make up airways.
But as far as the orginal question, if I turned up to look at an aicraft advertised as airways equipped I would expect to be able to take off there and then and do a full airways trip legally with an approach at the end. So all the kit has to be FM Immune where required, 2 x VHF/ILS/VOR, ADF, DME, 2 alts.
As IO says just sell it as it is and let the new buyer decide!