GPS can be used for primary nav in
all circumstances, bose-x
GPS happens to be the only way, in the GA context, of meeting the BRNAV requirement, FL095+ in Europe.
Sorry to be pedantic
The Dean - I can't speak for others but if I was buying a plane for IFR use I would hope to be intelligent enough to be able to work out what is needed to make it (a) legal and (b) usable, and if I needed to spend say £5k on fitting an ADF from scratch, I would take that into account when looking at the whole deal. I don't think the statement "airways equipped" is going to make a better price, as opposed to listing the actual fit.
In general, money spent on avionics is not really recovered when selling, which is why it pays to buy a plane on which the previous owner has spent all his money