I'm a lefty also, but I actually found most off the peg Beretta 686/687's fit me perfectly well, in fact just as a well as a proper left handed model I think.
Same goes for Miroku, Browning (apart from a Browning Medalist I shot, the comb ruined my cheek after a round of clays, shame, as I shot well with it!), Lanber and Laurona.
Couldn't hit a barn door with a right hand Bettinsoli I tried, but it was comfy at least!
My point being, for a first gun, don't rule out a second hand right hander at the right price. If it were a rifle sure, case ejection, bolt operation; all better off being correctly handed, But on a shotgun, my own experience seems to be that a good proportion of manufacturers guns would fit me well enough off the peg no matter what the handedness.
I'd only ever shot R/H guns for about a year after starting with clays. I tried my first lefty gun and I expected some sort of supreme comfort, picking up a few extra birds I'd have ordinarily missed and just getting that 'at home' feeling with the gun. But it never happened!
Some people I'm sure will be different, but for me as a lefty, R/H shotguns seem fine and are easier to move on if you want to.