Chaotic Crossbow is great for tanks. To start doing Dungeoneering, you go to the boat behind the Al Kharid bank, it will take you to Daemonheim, from there you walk North I believe there is a tutorial now on how it's done. You don't need any quests to start it. I got 1-80 using the Paid script from RSBuddy in a little under 2 weeks on my 1 Defense pure, running it 14 hours a day. Since you're a tank, I would have someone with high Fletching and Woodcutting to go in with you after you unlock Complexity 6, and get you the best bow you can use, along with someone with high Smithing and Mining to make you the best arrows that bow can use.