toshiba c855d laptop help

restore discs would be 3 or 4 dvds, 8 or more cds. SATA hard drives for laptops are universal in install/fitment. The difference in price is in storage space. So a 500gb drive will cost more than a 200gb drive.
As for removing the original and installing the new, its pretty straight forward but to watch one or more of the many videos on Youtube about servicing laptops will help.
Your Toshiba model number is very likely on Youtube.