Bicycle Mechanics - triple to compact

I humbly ask your advice. My crank arm is jacked up (long embarrassing story) and needs to be replaced and I found a good deal on a compact.

What parts would be required to switch from a triple to compact crank? What would have to be done to the front shifters?

The triple is the stock bontrager select and the compact is a shimano ultegra on a trek 1200. Everything is pretty much stock on the Trek.

What would make the most sense: order a new identical crank, replace with a compact, or replace the crank arm with spare parts from a LBS?

In order of increasing cost:
1. Replace only the damaged crank arm, particularly if it's the left crank arm.
2. Buy a new identical crank since you can use the existing bottom bracket and no changes to the shifters or front derailleur will be needed.
3. Change to a new compact cranks with the required bottom bracket and, possibly a new front derailleur.

Shifter compatibility will depend on the model shifters you have. 9-speed 105 and Ultegra STIs were both double and triple compatible. 10-speed 105 is both double and triple compatible but Ultegra is specific to one or the other. However, a triple STI can be used adequately with a double crank by locking out the granny ring with the fd's inner limit screw.

If you have an external bearing bottom bracket, you're good to go with just new cranks. If not you'll need to go with a new BB, 109.5 length (ie; if it's Octalink). Shifters are fine, ya might want to drop down the FD some.

Why would you replace with a compact? Do you really want worse gearing?