Both my wife and I are left handed, and relatively new to the shotgun sports (2 years). We have struggled hard with this exact issue, and have tried several iterations of trying to get used to guns set up for right handed folks. We prefer over-unders to semis (the stock fit problem is pretty easy to fix with how the semi's all have adjustable plates for casting left or right and adjusting drop), so our focus has been on the O/U guns.
Firstly, cast on/off are infuriating terms, as they are always referencing a right handed shooter. Ergo, a gun with cast off moves the cheek piece away from the centreline of the gun (away from your cheek) relative to a right handed shooter, but for a left handed shooter it moves it inwards (towards your cheek). Our experience in being left handed and trying to shoot a shotgun with cast-off was that you get slapped by the stock a lot, fatigued easily, and generally upset with with the gun.
Secondly, because we liked the style of the beretta over unders so much, we've tried on multiple occasions to have their wooden stocks bent by professional gunsmiths/gunfitters. Our experience here is that the wood stocks return back to their original shape over time, and require multiple sessions of bending and adjusting, all at reasonably high cost. This was very frustrating.
So, our solution was to migrate to the browning guns, specifically the cynergy. Both the 725 and cynergy field guns have neutral cast, and their synthetic stocked guns have a readily available easily replaceable cheek pieces that allow you to go either cast on, cast off, or neutral. W've both got our personal guns set up with the Cast On cheek piece (which browning marks on the cheek piece as LEFT), and we could not be happier. My wife now easily shoots a 12 gauge cynergy sporting synthetic, and she is giving me a run for my money in the score department! Her former silver pigeon 20 gauge has been re-named "the Wifebeater".
Good luck!
Cheers,
Brobee