There are only a handful of lefty semi autos. I'll list out some good righty models that I like by price range. I'll point out some features and pro's and con's for you. I would stay with 12 gauge 3" gun as they are more versatile for clays and hunting. A 3 1/2" model is a good choice for a waterfowling/turkey only gun.
$750.00 and up: Benelli M2 is Inertia operated. They are light in weight and very easy to clean. The have an automatic magazine cutoff and shims to adjust the stock. It has a good safety shape,size, and location. It holds four shells total and more if you add an extention. Some tend to think this design has more felt recoil than a gas operated gun.
Beretta AL391 is gas operated. They are fairly light and have very thin forearms. They have an automatic magazine cutff, bolt lock back lever, and shims for the stock. It holds four shells total and can not accept an extention.This design is more complicated to clean than most.
Browning Gold is gas operated. They are fairly light and pretty easy to clean. They have the speed load feature, stock shims, and a magazine cutoff switch. It has a good safety shape,size, and location. It holds five shells and more with an extention. The forearm/receiver juction could be much better than it is. There is a $100.00 rebate on all Golds to the end of Nov. The Evolve and Fusion (Gold) models are similar to the standard Gold but are very light in weight and balance with more weight towards the stock.
$500.00-$750.00: Beretta 390 and 3901 are gas operated. They are basicaly the same gun with different shaped receivers. They are fairly light in weight, have stock shims, and automatic magazine cutoff's. They each hold four shells and can't accept extentions. They are not too bad to clean. The 390 only comes in B/S with a 28" barrel in 12 gauge.
Beretta Pintail/ES100 is inetia operated. It's fairly light and has a slim forearm. They are very easy to clean and has an automatic magazine cutoff . It holds five shells and can't be extended. Shims are available for this model. This model does not have a magazine release making the unloading take a tad longer than most.
Franchi 612 and 712 are gas operated. They are the same except for the receiver shape.They are very easy to clean and light in weight. They have an automatic magazine cutoff's and shims for the stock. They hold five shells and more with an extention. They don't have the best triggers in terms of weight and creep.
Weatherby SAS is gas operated. It is fairly light in weight and very easy to clean. It has a magazine cutoff switch and shims for the stock. It has a good safety shape,size, and location. It holds five shells and more with an extension. This model was made by SKB first then Valtro for Weatherby. Parts do not interchange between the two.
Winchester SX2 is gas operated. It has decent weight and is pretty easy to clean. It has a high rib and stock shims are available. It hold five rounds and more if extended. This is the only model that does not have a magazine cutoff of any kind. The Light field model is the same but quite a bit lighter and balances with more weight towards the stock. The SX2 models have a $100.00 rebate to the end of Nov..
Barrel length is very subjective and each model will have a different feel with different lengths. A barrel from 26" to 30" should work well. Once you decide on a model try different barrel lengths on it to see which one you like the best.
Look at as many as you can and pick the one that feels the best to YOU.