Yes, you can start with the tummy tuck and later add the back component. Many times, this may be preferred to avoid such a long surgery and increased risks. Sometimes the lateral or outer areas are revised when the back lift is done. I would suggest discussing all your options with your surgeon.
Yes it is possible to do your surgery in stages. It is an easier recovery to do the front and back separately. It is true the costs will be higher but remember that complications increase with long surgeries. Good luck