It's one of those things that would not move the needle on sales even .01% too. New customers don't care if we add more zombies.
That is only partially true. Directly speaking, no, more zombies don't move the sales needle. However, more zombies are definitely one of the more requested items and adding more will increase enjoyment of the game. That leads to better reviews. And better reviews do impact sales.
Keeping in mind that I have played less than one year, I may not be "new" but I am also not really what you are thinking of when you think of veterans. And it didn't take me very many hours of gameplay before I noticed the repetition of the zombies. I certainly would have preferred more right from the start. Unlike a game like a military game where you expect everyone to look virtually the same because the military doesn't exactly allow variety in clothing, this game is based on regular townspeople and so the expectation is that there will be variety. You can't assume variety doesn't matter to people just because they are okay with lack of variety in games where it makes sense. From the many requests for zombies over just the past year that I have seen here, it is something people want and so is an important thing to consider. If it can't be done before gold, there is nothing stopping you from planning for it following gold.
Regarding limiting features so it works on console, one of the worst choices a game company can make is to do that. I get that there is an expectation that PC and console will be identical, but that is really a dumb notion that isn't a requirement. PC will almost always be able to do more than console and limiting the game just because console can't handle someone makes it a worse game for PC players. These days, game companies make games for both PC and console and try to keep them the same and it just lowers the quality of the PC game significantly. I'm not saying to have major features on PC and not console, but if you have to limit something like zombies or painting textures on console, go ahead... But don't limit them for PC. Just be clear when you sell the two that PC has a few more things. This is really no different than allowing mods on PC even if you may not have that option on console.
Last, I am pretty sure most DLC sales come from veteran players, or at least those who have played a decent amount. I could be wrong about that, but I have only once bought a DLC for a game I hadn't played enough of to know if it was worth paying for the DLC. So keeping your veteran players rather than losing them due to boredom of the repetition of things like zombies will also increase sales in the form of DLC sales, which you have said you were looking into doing. So, yes, sales are impacted by whether or not zombie variations are added, either in the form of new unique zombies or skinned zombies or both.
If we could get dozens of new unique zombies, I would obviously prefer that. But adding only a couple isn't going to really make any difference. That is why I mentioned skins. It's far less work and allows for adding far more variety. Yes, it's not the best option but it does give you far more variety than you'll ever get from only adding unique zombies. Some of the comments about how it skins aren't good are in reference to skins that are poorly implemented. They don't consider that skins can actually be implemented very well if the developers choose to make the effort. Just because some games are designed with skins and don't put any real effort into them and so they look horrible doesn't mean you can't have them look good with the appropriate effort.