The PvP isn't exactly amazing in this game but if you just want survival and teamwork then you'll love 7 days. The hordes spawn in intervals of specific days as well as wandering hordes that are more/less frequent depending on the biome you're in, forcing you and your friends to manage your time and resources efficiently. I play solo personally and find the game incredibly difficult even with 500 hours on it and 'Zombies Run' turned off (because sprinting zombies don't make sense to me personally).

The building is really cool because although it takes a bit of getting used to; the structural integrity system makes building a real challenge and if done incorrectly your entire building can collapse either under it's own weight or due to losing some support because of zombies attacking your pillars/walls/etc (You can build Metal Trussing to support your structures, though they take many Short Iron Pipe to build which are made from Iron Ingots). In order to build some of the stronger materials (concrete, steel, cobblestone), you'll need to get a Forge ASAP and try to locate the respective recipe books, as well as a mine (which building inside of can be really terrifying since you can trap yourself and have to dig out a second exit), but once you do you'll be able to get the resources for a much stronger structure, as well as a Fireaxe, Pickaxe, and Hunting Knife (also prioritize making a Sleeping Bag which take some Cloth Fragments that you can get from destroying Military Tents or Curtains, because otherwise if you die you'll spawn randomly which can end up as far as 15km away from where you were).

Pickaxe and Fireaxe simply need Iron Ingots to build, so you'll need to build a Forge, which requires Bellows (8x Wood,1x Short Iron Pipe,12x Animal Hide) to build, and needs basically the same things on top of that to make the Forge itself. To get Animal Hide you can craft a Large Bone into a Bone Shiv which you can use to harvest Hide/Meat from Rabbits/Deer/Bears once you kill them (or once you find the recipe for a Hunting Knife and have Iron Ingots, you can make a Hunting Knife and harvest dead animals much quicker). Zombie dogs do give hide but you'll also get Rotting Flesh from them which you'll have to drop otherwise you'll attract zombies, and if you need Feathers in bulk for Arrows you can always harvest Chickens for them.

Crafting guns/ammo is a pretty difficult thing and if you're playing solo can feel impossible having to manage so many resources, which is why I personally convert a house or 2 in each town into a Storage/Outpost so that I can travel safely between areas throughout the week and hopefully find Gun Parts/Guns/Recipes/Tools/ etc (and with the new Waypoint system you can keep track of them easily). Guns become more important in the Industrial City type biome, because Cop Zombies projectile vomit acid that can destroy blocks quickly and they go berserk on low health, essentially making them feral, and Zombie Dogs are an incredible nuisance.

I know there's a lot to take in there, and there's still more to learn, but I think the game is incredibly good: well worth the money I spent on it. The game can be frustrating at times but for the most part the annoying things will be due to your own mistakes, which is why I tried to cover what to avoid rather than simply explaining how to bypass/exploit everything.

Hope this was informative and good luck!