http://www.coinopexpress.com/ has adapters for using arcade lightguns on a Dreamcast (as well as arcade wheels to DC, and DC to Jamma adapters). You'll have to talk to a salesman to get more info on them, and don't expect it to be less than $50 (much likely more) for the adapter. Since you're willing to buy the expensive Happ gun, I don't think this will be a problem for you. I think they also have PSX to Jamma adapters, so maybe they have arcade guns to PSX adapters as well.
As far as lightgun games, House of the Dead 2 is pretty much the best commercial lightgun game out of the few the DC has. However, the Dreamcast homebrew scene is just starting to tap the potential of the gun. There is DreamSNES (SNES/SFC emulator) that supports the gun as a SuperScope (problem is most of those games are slow with sound), and a version of MAME that supports the lightgun (only a handfull of games right now, and no sound). More info on
http://www.dcemulation.com (note: that site has some pop-up ads, so start your pop-up blocker when you go there).