Pick one, whatever is your best shot, get that going then consider options for the others.
I think, that in your shoes, I'd let a shop evaluate the fork, and rest of the Dahon's condition,and if all is OK but a front wheel and be good to go. This has the advantage of being easier to mount with a bad hip or leg.
Otherwise, add a front brake to the coaster bike and get that rolling. Leave the rear wheel alone and you can add a 2nd brake whenever you feel like it. You can also disable the brake feature of the coaster wheel. We used to do this routinely, and any decent mechanic should be able to do it for small $$$ if you want (I wouldn't bother unless it became the #1 bike).
As for the 1st bike, since it needs the most work, I'd leave it for last, or donate ot trade it to a co-op for some labor on the other two.
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