I was told once by the home builder that many lenders require at least basic flooring before closing the loan. This means that you have to remove the basic flooring later on and replace it with upgraded materials. I asked them if I can ask my own hardwood company to install the floor if the lenders approves him. The builder agreed and gave me a credit for the flooring. We saved about $3k.
I would recommend installing the flooring before you move in to avoid the hassle of moving your furniture around.
As for the kitchen I saved about $5k by using my own company instead of the builder’s.