Nayiri Doudikian-Scaff, MD

You need to see a surgeon, have them evaluate the skin to know if tummy tuck or something like smart lipo (that tightens the skin) is better.  Remember that a tummy tuck leaves a long scar (which fades over 12 months).

Breast implant size selection requires measurement of breast base diameter.  A moderate or high profile implant would be best.  High profile for larger volume.

It depends on the baby's weight.  No heavy lifting of 20 lbs or greater for six weeks after surgery.  Your baby can be placed on you chest for you after about two-3 weeks when discomfort from surgery is not a limiting factor.

I recommend Tata Taimer bra from Lululemon Athletica.  Usually I have my patients wear them 24/7 for six weeks.  You have to purchase the right size.36D, 36DD, etc.  (whatever size your surgeon thinks you'll be).  For scar, you start treatment once wound is healed and no scabs remain.  Silicone gel sheeting or Biochormeum.  Usually scar treatment is started 3 weeks post surgery.

Yes, 12.3 is not that low.  Consider smartlipo which has less bloodloss, first.  Let it settle down 6-12 weeks later do tummy tuck.  Smartlipo is done in the office with local anesthesia, fast recovery, minimal down time.  Then when your body is recovered have tummy tuck.