You've got an Octalink BB. You can still get Ritchey compact cranks for Octalink on Ebay for under $100.

If you need even lower gearing, you can either get a 12-27, or for super low gearing, a 9sp MTB cassette and a MTB rear derailleur. If you want something in between, then you can get say a 12-27 cassette, remove the 16t cog, and add a solo 30t cog. Or make a custom 12-28 from a 12-25 and a 28t cog. Harris Cyclery sells the solo cogs. You need a steel or Ti freehub body to do this, aluminum won't handle the torque.

If your chain wrap (big ring + large cog) - ( small ring + small cog) is about 31 then you need the longer cage derailleur.

The 9sp Ultegra shifters were all triple, even those sold on double bikes. Does anyone know if the same was true of 105? If that was the case then the shop should have told you, but you never know...

No need for pen and paper to make a gearing chart. Use this:
http://www.sheldonbrown.com/gears/