Its gonna go up the stairs no matter what the weight is (lets be practical here
). Just be sure that your center of gravity is low. That way when you do climb up the stairs the robot wont tip over.
Just a FYI, our robot is 90 lbs without an arm and it gets up the stair just fine.
Also when you use metal you frame will probably be .2 times heavier so no matter what you do it will be heavier.