I'm glad I'm being helpful, I don't mind answering questions based on personal experience.
If he's a tad nippy from not being handled often I wouldn't think that he won't calm down with regular handling. Though some kings are just nippy by nature, I would imagine that his guy could tame down into a real nice snake with time and patience. Be prepared to be musked as I'm sure he'll do that often: even my King who is "puppy dog tame" musks me from time to time (it's not a big deal - it washes right off). I would handle him as much as you can before you devote to taking him in/adopting him to see how you mesh with him and how comfortable you are with handling him before you take the plunge with this particular snake.
Who said you can't feed in it's tank/tub? I don't really agree with the whole "it will become aggressive and try to bite you if you feed it in its tank/tub as it will see your hand as food" thing that some (not all) people believe. I feed mine in his tub and he is perfectly fine when it comes to reaching in and grabbing him for handling. If this one that you're considering is showing signs of cage aggression (striking at you when you reach in) then you can work with him to overcome that (using a snake hook would be one way). You may end up getting bit a few times but those are minor at best so that shouldn't lead you to believe that you should give up on him (if you get bit don't put the snake away right away as they will come to believe that biting you is an easy way to get out of being handled). Perhaps watch how he reacts when the current owner goes in to handle him also: he might have no cage aggression at all.