Zombies4Test is a great set of zombie-themed monsters for any MTG-based game.
Walkers, sprinters, crawlers (who can get in through one-node-high openings!) all work nicely.
I'd suggest disabling the tanks and spitters at first, especially in singleplayer, as they have massive health pools and will easily overwhelm a lone player with starting gear. Fortunately, disabling them is as simple as unchecking their box when enabling mods, which is very nice.
I also like how they vary their movement, sometimes shuffling to one side or the other, as it makes it harder to target them with ranged weapons than if they were to just move straight towards you..
You'll need to build a base quickly, and fortify it to keep the nasties out -- they can break doors and windows, so beware! I've even had one manage to follow me up a ladder, which was a fun surprise! All in all, highly recommended if you want some threatening monsters in your game.