Well the items at the top are just basic all-around items that don't necessarily adhere to the situation. The items section covers some of the options available, but everyone has their own play style. This guide is more of a marker. It would give you a place to start with Warwick and then if you don't enjoy it you experiment to find your best setup.

Guides are Guides, not rules. They can be followed or not, but that isn't my choice.

I still don't really agree with your main item build. But I'll give you a +1 in order to draw some attention to your build. And you seem to be willingly to improve your guide. So I still don't have lost hope with you. ;)

Congratulations for your first guide. I know you put a lot work in it but I don't agree with it completely.

At first: Why don't you mention Sorcerer's Shoes? In my opinion they are the best choice for Warwick as your ultimate and your Q deal magical damage (I neglect the minor physical damage your ult deals)? Ultimate + Mardred's + Sorcs can deal a lot damage.

At second: It seems like you want to make Warwick a fulltank with some damage. I don't agree with this attitude. Warwick can't replace a fulltank and you should notice that. Warwick can't engage well. If you plan to use your ultimate for that most enemies will just stun you. Warwick does not have serious CC.
Warwick is tanky, no doubt. But he is not a tank. For this reason I would replace FoN with Wit's End. It gives you some more magic resistance without providing too much.
In addition to that I'm not a fan of Thornmail on tanky champions without taunt (Rammus, Shen) because there is just no reason for the enemies to attack you. There will most likely be more viable targets for them (support, AD carry, AP carry). You will either be focused if other targets are dead or when your health are low. And then Thornmail won't save you anyway and the team fight is most likely lost. Sunfire Aegis is imho a much better item (unless the enemy team has not more than 2 physical damage dealer).