Not sure why your changing from a triple setup to a double/Compact. You already have a multitude of gear ratios. The compact crankset isn't going to give you anything that you don't already have regarding gear combinations. It will probably provide some weight saving with the elimination of the middle chainring, shorter derailleur cages, and shorter chain. But I don't know if that is what you are after or if the net loss will be worth the expense.

Fortunately, the Ultegra STI levers are Double or Triple capable. So you will not have to change the levers/brake handles. That is an expense saved.

But the Front and rear derailleurs are Double or triple specific. The triples have longer cages. You will also have to adjust the chain length accordingly.

You should get the Bottom bracket that works best with the crank that you select. Don't let the bottom bracket decide which cranks to purchase. Pick the one that best suits your riding style.

I switched my wife's '04 Shimano 105 nine speed from double to a Compact crank. I purchased the crank and the associated ISIS bottom bracket and then just lowered the front derailleur a bit to accomodate the smaller diameter chainring. The braze on should have a slight oblong hole that allows upwards or downwards adjustments. The chain should be shortened slightly but I did not do that and have yet to experience any problems. Once she puts on some more miles on the new configuration, we will see if that needs to be revisited.