I have 3 commercial sealers here and I am absolutely tickled pink with them. They are one of the top investments we've made, have been an incredible benefit to the business, and paid for themselves within a couple years.
Get the biggest you can afford and have the space for, and then go just a hair bigger. In this case, bigger is absolutely better. We started smaller they we should have and quickly upgraded to a 2-bar 22" machine and later a bigger 2-bar 26" machine. I can't strongly enough recommend 2 burn bars if you plan on doing any kind of production. I'm really small potatoes and during hunting season I still run 2,000+ lb of sausage a week through the bigger machine. I can't imagine the bottleneck a single bar would create.
Also, one thing we didn't think about is lid shape. The machine we have in our lobby has a clear lid with a flattened dome shape to it, with a flat area about 2" wide right behind the bar. It works great for things that aren't much above the level of the burn bar, but as soon as something stands up higher (like a 2 1/2" summer sausage casing) is pushes the product back away from the bar and leaves more excess plastic on the outboard end of the bag than I like.
Another thing- make sure you have 2 burn lines per bar. Each bar should have 2 heating elements to it, one for sealing and one for cutting off the excess bag. I had my guy readjust the second element to do a double seal. I'd rather have a more certain seal and spend a half-second with scissors trimming the bag then have to waste the time of re-bagging something.
Nitrogen injection.....eh, personal preference. None of my machines have it and I haven't seen any difference at all. Anaerobic environment is anaerobic environment. I don't ship, sell everything in-house or at a farmer's market and oxidation hasn't been a problem in 24 years. We vacuum seal all the fresh meat in the fresh case and it gets a little darker, but maybe one in 50 (or more) customers asks why the meat is darker then the stuff at the grocery store. For cured products, the color never changes, unless it's a response to light through the clear vacuum bag over the course of a couple months of an item sitting in the coffin cases.