Much the same as above but get a SIG 357 instead of the .40SW. Still get the 9mm conversion barrel and two 9mm magazines. If you want to get the .40 barrel later that is good too and uses the 357SIG magazines.
It is a delicate subject around here but 357SIG is probably the best defensive round below the 10mm and is almost certainly better than the .45ACP both in effect per shot and rounds per magazine as well as pistol weight. The 10mm G20 is a very nice pistol and you can get .40 and 357SIG barrels for it but not 9mm. If you really want the .45, try to hire one first because the grip, like the 10mm is too big for many hands.