How low a gear you want will depend on where your tour is going to take you and how strong you are. That said, I'm not a particularly strong cyclist, but was able to get across country using a Sugino XD triple crankset (46-36-24) with a 11X32 cassette. This crankset has a 110/74 BCD, and there's a good selection of chainrings available for this BCD. It lets you get a larger chainring than is generally available with a compact mtb crankset while still getting a small inner ring. I'm using a 105 bottom pull front derailleur with it and it works fine. Have fun on your tour!