Would like to remove all the fairing, install a new seat, and new headlight. How difficult is all of this?
. . . is there anything wrong with starting with it?
If you can do research, follow instructions and have patience, you should be able to complete your project. Countless threads here and elsewhere show various ways of doing those jobs. The bikes you have shown as examples do not have their original speedometer and tachometer instrument clusters. They have aftermarket instruments. If you choose to replace yours, you will be learning some basics of wiring and incur some additional expense for adapting the new instruments to the bike.
As has been indicated already, you should look through the Custom bikes section here to get a feeling of what can be involved and some of the obstacles to overcome.
If you buy that S, you should have no difficulty finding buyers here for some, or all, of its fairing and whatever else you might not want.
Before you start the bike and run it, you should know the bike's mileage and how often it has been ridden. If it has been well and recently run, there should be no problem. If that bike has been in storage with fuel in its tank, you won't be doing it any favors by trying to start and run it because decomposed material in the tank might infiltrate the rest of the fuel system. If it has been in storage for a few months or more, you won't really know how "clean" that K really is until you remove the fuel tank's filler cap assembly, all its fuel, then shine a light into its tank and have a look around in there.
It looks pretty good right now though. :giggles