2.5 v6 out of a smashed ST24/ST200? I have recently acquired a ST24 for buttons and although many (on different forums) have suggested that it is not really a good project car I think it is worth it. I don't know what it is about the mk2, they are hideous looking but I love driving them. Mine however has rust in all the wrong places and 205k on the clock.
At the end of the day, the mk2 is never going to be worth much, it all comes down to how much you like the car and how much you are willing to put into it.
I am certain that someone on here will be able to tell you if the 2.0 zetec is tunable or not.