I'll be a tad bit contrarian.
Stay away from DA Colts, unless you know a gunsmith that is competent to work on them. Few are, anymore.
Primarily have in the case what you sell.
Outside of that, maybe a Model 10/64. A Model 14 or a 6" Model 19 would be nice, but they're getting pricey. Most of the range guns I see are some flavor of bottom-feeder, because that's what people want to shoot.