Indeed, 8 zerkers with the Champion are a good number, fluffy and efficient. I used 10 for a bit and found that I was overkilling stuff left and right and then staying exposed to fire on the field. With a few less berserkers, I can ealiably leave only 1-2 enemy models in CC into the next phase. They might kill a zerker or two back, but it's -way- better than being sitting pretty for plasma, vindicators and other stuff to shoot you up.
As for load-out, it depends. If the zerkers are alone, take a Power Fist. It's S9 on the charge, enough to crack even a Land Raider with some luck If they are serving as an escort to an HQ, however, like to pick a power weapon instead. Why? Kharn is already a glorious mech-nuking machine with 6+2d6 penetration and tons of attacks at I6. So a power sword to add 5 attacks at WS 5 and I5 will help you a -lot- at thinning MeQs and Terminators before they start killing back. they are particularly good at killing librarians who usually lack Invul saves.
If they are with Abaddon, also skip the fist. Take a meltabomb if you expect to fight Land Raiders, but 5-10 S8 attacks at I6 beat 4 S8 attacks at I1 anytime.