For the fenders, you need to remove the rivets that hold the cheap L bracket on, and replace that L bracket with some sturdier metal. Or you can get a fender mount kit from Wald that nests right up under there, if you're not much for the fabricating.
Actually, looking at that, I'd still add a sturdier L bracket, although the little cradle underneath looks handy.
At low speeds, I don't think a coaster-only is a problem, really. But the coaster on that bike wasn't really designed to stop a bike going 30MPH. I've heard stories of coaster brake arms bending, and if your pedal chain goes for some reason, you don't have any brakes at all.
With front and rear brakes, most of the stopping is done by the front brake. But you do want a rear brake, too, so that you can control your stop, rather than go flying over on your head.
I think you should be able to find a decent caliper brake that fits around the fender, and can share the same mounting hole in the frame and forks as the fender does.