Basic Action Super Heroes!

I had an idea today for a character based on an old Japanese figurine--a girl with parts of a Sherman tank. Most of the tank parts are concentrated in the legs, so I was thinking of her having lost her legs in an accident years ago. In her secret identity she's in a wheelchair, but puts on her prosthetic legs when going out to fight crime.

I didn't want to build her in the manner suggested in the description for Physically Challenged (Agility 0 with Boost for upper body) because when she's being heroic she'll need the use of her steel legs. Would it be acceptable to build her with Normal, and define Physically Challenged as giving her Agility 0 with her legs in her Normal form?

(As an aside, when you build someone with the Normal disadvantage, does it limit the number of Physical Skills they'd otherwise get?)

Give her the physical disadvantage but also give alter ego to reflect that when she "transforms" into tank form she is essentially a different person.

Interesting idea, though difficult to justify--she's still the same person in terms of mentality while wearing the legs, so it's not as if she's changed as much as the Hulk. Here's what I have for her so far:

TANKER

Billie T. Sherman was the fourth generation of a tanking family whose prize possession is a Sherman tank from World War II. She lost her legs in a tragic accident, so was resigned to never being able to drive her family legacy. However, her grandfather built for her a set of prosthetic legs modeled after the Sherman tank she so loved! She used those legs, as well as the turret-shaped gun also provided by her grandfather, to stand firm against crime and injustice! She is the talented Tanker!

25 points

Brawn 3
Agility 3
Mind 2

Total Stats: 16

Powers:
Armor 2 (x5 Soak)
Special Attack 5 (x6 Damage Multiplier, x4 Hit Multiplier)
--Range 3 (60 squares)
--Gadget: Easily Taken -1 (100 Hits, 50 Soak)

Total Powers: 9

Physical Skills:
Athletics/Throwing
Drive/Steering, Control

Mental Skills:
Military/Gunner
Technology/Repair

Advantages:
Mentor (Grandfather)
Never Surrender

Disadvantages:
Normal (Brawn 2, Agility 2)
Physically Challenged (Paraplegic; Agility 0 with legs in Normal identity)

Mental Malfunction: Overachiever—She's determined to prove there's nothing she can't do by doing everything!

Hits: 100

Height: 5' 3” (with prosthetic legs)
Weight: 95 lbs. (450 lbs. with prosthetic legs)
Eyes: Blue
Hair: Blonde
Age: 19