1. If you're using brifters, go for the Tiagra front derailleur and the 48t big ring. A 42t big ring will look funny and not shift the greatest.

2. Since you haven't bought your crankset yet, ask the seller what BB spindle length it needs. If they tell you something like 110/113, use a 110mm spindle with 130mm dropouts and a 113mm spindle with 135mm dropouts. The test fit criteria is the middle chainring should line up with the middle cassette cog. A millimeter or two off in either direction won't generally be a problem.