This seems like an expensive idea unless you already have a nice 120V power supply. If you do have a source of nice steady power, why not simply rewire the entire head for 120V? You'll have a nice clean install and one less transformer to start buzzing as time goes on.
Timer? Every time I look at building a timer, the difference in the cost of buying a brand new one always seems less than my time. I have always thought that a PLC would be the easiest way to combine all the jobs required to control an enlarger and they are really cheap at automation direct. But whenever that idea pops up, I remember that my enlarger works just fine the way it is. <G>
In any case, I actually hope that you build a really cool control system and then post it on the forum.