Welcome, and that's certainly a fine grab-bag of lenses to put on a future Pentax DSLR. I would echo everything that's been said about the K-50; I would have to be wary about buying it knowing what I know and given the fact that it has been discontinued, but ultimately the chance is yours to take (and if I were going to buy one, it would be from an authorised seller or store, with a full, active, valid warranty).
However, all the lenses are usable on whichever Pentax DSLR you choose to buy, and I would not be in a hurry to get rid of any of them until I'd given them all a fair try. I suspect your grandfather would be pleased to see his old gear go back into full time use again.
The advantage of the current line of kit lenses is that they are weather-resistant to go with the current lineup of cameras. All of those are weather resistant EXCEPT, please note, for the KS-1. They didn't WR that one for some reason. For the sort of photography you do this might or might not matter, and please note that none of your older lenses are WR either, so consider that in context. Having either component non-WR will render the whole combination non-WR.
The F and FA lenses, particularly the FA, are autofocus, fully functional on a Pentax DSLR and can be used to the limit of their capabilities. The FA, I think, communicates extra information to the camera body that the older F does not - but I could be wrong on that one. Either way, both series are perfectly usable and there are those among us who are more enthusiastic about the F series for various reasons. To each their own.
Best of luck with your camera purchase!