5. What are the "must buy" items for Anub'Arak and why?

A strong lane with NA and Lina.

Whisper: Dagon to further increase his instant damage output, necrobook does the same in a different way

Loda: I think a must buy item for NA is mostly Dagon, but only if you can get it fast enough. Atleast before 30 min or so! After that point I think the possibility for NA to own is a lot lower.
Necrobook is a possible item vs some lineups like Skeleton King, tree and so on, vs heroes that relies much on their manapool but haven't got so much to begin with

zenition: As I mentioned before, the strength of NA is his amazing burst-damage output, he is one of the few hero's in the game that can kill another hero almost instantly, and with a dagon(1-5) he stays useful throughtout the entire game.

 
6. What's the basic strategy and gameplay for Anub'Arak?

Whisper: The strategy for NA is to annoy the other team as much as possible. You can do this by mana burning them in lane and then leaving the lane to gank once you hit level 6 you can pick good points to try and pick people off but make sure to get some farming in, because if you fall behind in levels and items you'll quickly become completely useless

Loda: The basic strategy of NA is to early on either play on a defensive lane where you help one hero to farm etc by manaburning the opponents. He can also be placed in a really offensive lane like NA + Lina or something like that. After some earlygame ganging etc he should put some time into farming a dagon imo. Many players keeps ganging and just ends up with no items at all, you need to know when farming is the right choice to be stronger later on. When you got a dagon you should try roaming around getting solo gang kills and just keep on doing that the whole game. NA is very good together with Zeus and Furion imo.

zenition: The basic strategy would be to kill as many heroes as possible. In essence, this is what Nerubian was designed for, and what he does best.

 
7. Do you have any tips for a newb Anub'Arak player? Such as when you should stun and when you should not, or when you should use your ultimate?

NA uses Vendetta followed with Impale.

Whisper: NA's ultimate makes a very distinct noise. Make sure to use it out of "hearing" range, and make sure you actually HIT the enemy before casting impale, and then mana burn.

Loda: hm I don't really know, some classic tips are that you should use Vendetta when u tp to a tower. Press tp on the tower and then hold shift and press v, this makes you arrive invisible to the tower and gives you an easier way to not get detected by the enemy hearing you go invis or something like that you always have to practice so you can execute all skills fast enough! If the enemy has time to react it's possible that he will survive! Be fast with Vendetta then impale manaburn + dagon, don't leave any chance of surviving

zenition: My best tip for a new NA player would probably be, not to forget to kill creeps. It's easy to forget when you're playing a hero with this much killing material, to forget that money for a potential dagon, and levelling is just as important as killing, when playing Nerubian Assassin. If you get Dagon + level 11 quite fast, NA will be able to kill most Agility/Intelligence heroes solo.