If you go the closed head drum and Weber lid route, keep in mind that you can grind almost 1/8" off the outside of the top bead without compromising the structural integrity of the drum. That way, you may not need to flatten the lip of the lid and re-shape it.
If you go the open head route, keep the locking ring to ensure a tight seal and to keep Lookie Loo out. Get the kind with the 2" hole and just put a 6" pipe nipple in the hole. In any case, stainless is definitely preferred.
I have found the Sears and KMart kettle lids to be almost identical in diameter to the Weber. The WalMart and Lowe's lids are too small and the vents are awful. Haven't checked the Home Depot version yet.
I got one of the cast iron grates for Christmas. Awesome product! Keeps me from having to "steal" the grate from my Stok Tower grill when I want to sear something.