This model has 2-1/8 in. fully glazed trap-way.
Yes, it is the Z5551-K.
The recommended seat for this model is Z5958SS-EL Elongated, white, closed front toilet seat with cover & stainless steel check hinge.
Toilets come in 10'' and 12'' rough-ins, yours should be a 12''.
This toilet does not include a seat
No, this product doesn't have other color option.