If you want to use the triplizer, you will need a longer spindle, but exactly which one will depend to some extent on the cups you are now using. Generally speaking, you'll get a good chainline if you measure the drive-side length of your existing spindle (you will need to remove the spindle to do this, of course) and find a matching spindle that's 6 or 7 mm longer on the drive side. If there's a good bike co-op near you they likely have a drawer full of assorted spindles for you to try.
Another option would be to replace the existing Swiss bb with a Grand Cru threadless BB, which costs about $50, I think. You might be able to sell your existing Swiss BB on ebay for that much if it's in good shape.

If you can live with a 37-tooth ring for a low--I'm guessing you have a 42 on there now--you could keep it as a double without having to make any changes beyond switching chainrings. Going to a freewheel with a 34 big cog at the same time would give you a sub-30-inch low--not far below what you find with a lot of triple setups.

I also like the 86 BCD cranks from Stronglight and others, although the smaller rings for them are not always easy to find.