I changed my Ultegra double to a triple.

The changed items were crank, bottom bracket & chain. I used a Sram chain, the STI was always triple, I kept the original double derailleurs with no problems.

I did increase my cranks to 175's while it was a free option, you might want to investigate your happyness with crank length if you make the upgrade.

You gotta have a BB tool.

The cassette is your choice..your weight, strength and the hills you ride define the low gear. I you need less than a 27 on the rear you are going to have to change rear deraileur to a mountian type.

With your bike, other than the BB, I would only upgrade to 105's.

But is is only my opinion...........

Terry