Your install should not be too difficult. If I understand you correctly, the basic set up would be:
-Tablet replaces the headunit
-Headphone jack of the tablet connects to amplifier
-Power connector of tablet connects to cigarette lighter
At the very least you will need: 1) Tablet, 2) Amplifier, 3) Miscellaneous cables (such as car charger for tablet, audio cables, audio splitter, power wire for amplifier, basic tools to install amplifier,).
Another aspect of the install is making it "removable." If you want a professional look with hinges and a powered motor, then that will cost a lot. Some people have fabricated other solutions like this guy: https://www.youtube.com/watch?v=Q65f84mjN_8
The fabrication portion will vary depending on what you want. And so will the money.
I didn't go into detail because I wanted your thoughts about these issues first.