You can get replacement parts for the Ex. from Seyco.com. I had an Ex and loved it, not going into a long story but I sold it, was eyeing a second used Hegner, but didn't get it, so Ex were not on the market at the time, I purchased a Seyco. Seyco is a good saw, there are a couple things I liked a little better on the Ex. The only reason I would hesitate to encourage the Hegner (my favorite saw, by the way) is if your are a beginner, the Ex is an easier saw to learn on and to use. For one thing you have the option of top or bottom feed. You would be able to decide which you like best for the type of scrolling you want to do. Hegner is bottom feed only. Hegner tensioning is also a little more critical than on other saws. Something you get a feel for over time, but there is a bigger leaning curve. The Hegner will be around longer than any other saw you would buy. But, used you do not know how abused it has been. But Hegner is a great machine. If I could only have one saw, it would be Hegner for me.
Of the ones you mentioned, personally I would go for the Ex and feel very thankful that i got to start out on one of the better machines.
I can't really speak to the King, never even had a look at one, but parts and repairs for it I think would be the same as the Ex. Seyco would be your best source.
Oh, the Hawk depending on the age may be bottom feed only and maybe some other minor deficits, I do not have any experience with them. I want one, someday, you can't have too many scroll saws.