Iron Will as a fighter feat isn't bad. Its way better than endurance. You can pretty much top out at 8 levels of Singh Rager. The last 2 levels don't give you that much. Every level of Singh Rager is 2 levels of barbarian like you said.
You could be a paladin, rager, templar, windrider, divine champion and have it all: top saves, immunities, dr, attack bonuses, smites, specialization, uber mount, mettle, turning uses and with a ring of evasion and armor of absorbtion you are set. Wrath and rage stack. Smite evil and smite infidel stack.