Title says all.
I wonder if i can level up my apocalypse spel(and other magic-heavy spells)l to 15 with it.

GZ!

For gems and another crafting material you must salvage items on the anvil next to Griswold. But you can see and use that anvil only after "Anvil of Fury" quest.

http://diablo.gamepedia.com/Anvil_of_Fu ... o_I_Quest)

"Trade stash" is shared between all characters and "private stash" is only for one characer.

Is there cow level in this mod?
If so - how can i get there?

I'm asking about this - because that one cow in town (Betsy) is rly suspicious.

I have 30lvl Rogue and i can't kill Diablo on normal dif. I tried to farm better eq. in cave from wirt's map quest, but i still can't slay him. I have ~325 dps, so i think that maybe problem is with my defense - how much armor should i have? Or maybe damage reduction is key to succes? Or big vitalit...

Yep - Angelic Sickle (sabre).