Discs (of midrange cable style) are useful in wet, slushy, muddy conditions but in the dry offer no braking advantage over V brakes. You will need a new wheel, brake caliper and cable. The existing controls should work OK. Under hard braking, the rear does very little work, the weight is moved to the front wheel and this does almost all the stopping.
Disc-compatable racks have a large spacer which moves the struts 1" wider each side. The luggage is 2" wider which increases air resistance and reduces the ability to squeeze through narrow gaps.
Adding up the pros and cons, there are very few pros.