The following just doesn't feel right

"Johnny boasted that he was buying not just one but two Ferraris"

Are the following better (and if so, why?)

(a) Johnny boasted that he was buying, not just one but, two Ferraris

(b) Johnny boasted that he was buying not just one, but two Ferraris

(c) Johnny boasted that he was buying not just one, but two, Ferraris

(d) Johnny boasted that he was buying not just one but two, Ferraris