For a more detailed explanation per weapon, I recommend this guide: The Weapon Guide for L4D2 By Shao

To simplify and to point out the most important elements:

The one thing I would say that has to ALWAYS be respected; It's the ammo in reserve per class.

I'll update this guide soon, to expand more in detail.

Each gun type has a set amount of Penetration Power. It might not sound as an important factor at first, but think of the times where you shot through a door or even wall to kill-off a Special Infected.
Here is a graph (made by Pinsplash) that shows how much Penetration Power can go through each material.
If you want a more in-depth explanation, here is the video source of the image: Bullet Penetration in Left 4 Dead 2 by Pinsplash

Special Infected are obviously and undeniably the main icons that turn the tide in every playhtough. They wouldn't be a threat if they die too quickly, right?

So here are the HP of each infected to have an idea of how a weapon will do against them:

I would say it's best to separate the model from the script, so people could just make models for it without having to make multiple versions of the mod.
Also, the attachments a gun has is important for both gameplay and immersion. For example:

• Magazine: Even tho having a 40+ bullets in an average M16's magazine doesn't make sense, Valve made it intentional for gameplay reasons (infected hordes). However, weapon classes tend to have a set amount of ammunition, and it makes less sense that a weapon has less ammo then what it should have.
  
• Flashlight: All guns must have them. Aside of following the design rule as the other guns (melees have a static flashlight), reloading actions and the laser sights are affected with the flashlight placement.
  
• Scope: This will always signal that the weapon has a Zoom function. There are exceptions; Maybe it's a weapon from a game that features a scoped version of a gun?
  
• Accessories: Keep in mind that in-game lore, the survivors would find common well-known weapons left by either civilians or authorities. Would be kind of odd and funny that there are multiple guns with grenade launchers, bayonets and like 3 different scopes, straight out of modern Call of Duty... And that are unusable, by the way.
  
• FOV: Would look weird if a gun looks further, closer or even more to the side than the rest of the weapons.
  
• Camera Shake: There are games that feature camera shake during reload animations, but L4D isn't one of them; so aside from having a gun with that feature making it look out of place, there's the possibility of motion sickness from players.