1.) This is a 1989 Cannondale SM2000, is that a good derailleur?
2.) Will these dropouts suffice for a coaster brake? What if I over-shorten the chain? Do-able?
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A derailleur (or pulley-type chain tensioner) can't be used with coaster brakes. You also have vertical dropouts, so if you want to run something without a derailleur, you need an eccentric hub (which I don't think exists with a coaster brake -- the only one I'm aware of is the White Industries Eno) or eccentric bottom bracket (Phil Wood makes some). You can't just shorten the chain. You'll also need a new wheel on top of that (or at least a new hub) so that you can have a coaster brake. All in all, putting a coaster brake on a bike like this is not a good idea.
As for the bike being any good as is, that's in the eye of the beholder; it looks like it was a decent bike when it came out, so assuming everything is in working order it'd make a great commuter, for example.
If you really want a coaster brake, sell the bike and get a frame with horizontal dropouts.