Sirloin will no doubt be tasty, but in my experience, for the best pulled beef, chuck roast isn't to be beaten. I've done chuck roasts in the 6-9 pound range, and also done a chuck rolls, the biggest being 22 pounds, and they have all came out really tasty. The roll is the primal cut (the biggest piece you can start with) that makes the chuck roasts. I've found that sirlon is good for thin slicing like to make a roast beef sandwich, depending on how big your sirloin is. Either way, you'll have tasty stuff.

I post heavily on the Virtual Weber Bullet forum, and there is a ton of good info on there for smoking too. Smoke Ring is also a great place to start.

I have a couple of the little chief Smokers, they are great for cold smoking, and jerky. Then i have a built like a brick electric. and that thing is crazy for hot smoking. The trick to canadian bacon or any other pork product is the cure moreso then the smoke. Unfortunatly i have yet to try it myself.
Good luck! Smoke Curing is a fun and addicting thing! soon you wil be into cheese and all kindsa crazy stuff!