I offtanked Karazhan for my first run. I was in a similar thought process as you, so I asked the raid leader what I should do. I stayed in my Arms / Fury spec (I actually did respec to try out a good PVE dps spec, I'll probably respec again next Kara run as I think I'm going to MT it). I have Itemrack, so I can swap tank and dps gear with a single button press. If I was asked to offtank, I would look at the mob and decide what gear I was going to wear. If it wasn't a boss, and it was just a 2 pull, I would generally stay in my DPS gear and use a sword and board. Then again, I have a lot of HP in my DPS gear, with buffs I was over 11K HP. Sometimes the mobs I was offtanking were harder hitters, so I would swap into full tank gear. Others, like the Wizard of Oz fight, I was asked to pull Toto off the healers and I didn't even use a shield on that fight.
Ask more knowledgeable people how tough the fight is. If you don't feel like you have a good tank or dps set, then I would put on my tank set for every pull I offtanked.
As offtank, you can be expected to offtank Moroes, because he will gouge the tank and begin attacking whoever is second on the aggro list. So basically you will fight for aggro the whole fight. A druid tank would actually be better for this, as they can get some great snap aggro and they get rage more easily by just attacking. You will also be expected to offtank Attumen. Basically it's a dude with a Horse. Main tank starts off on the Horse, then Attumen appears. Offtank picks up Attumen until he combines with the horse, then MT picks him back up and you finish the fight by dpsing.
There'll also be lots of trash pulls where you offtank mobs, in case you don't have enough DPS. When you are tanking anything, always stay in defensive stance. You gain way more threat in defensive stance.