Go stainless, if your budget allows for it. If not, then aluminized steel is your only other option.

Some higher end shops have mandrel pipe benders. Most shops however will not. Mandrel bends are far superior to traditional pipe benders because mandrel keeps the diameter of the pipe the same whereas traditional bends slightly decrease the diameter in the bend.
YES most kits that you buy from a supplier will be mandrel bends, but not all. I would ask the shop and the supplier of kits if they have mandrel bends before purchase. Personally I think that if you're going to shell out that kind of coin, you might as well get your money's worth.