can I use an old Toshiba Satellite, and build a modern laptop?
Can a 2002 Toshiba Satellite P15 be rebuilt with current state of the art components? Is it cost efficient?
Yes and no. You can do some upgrades like faster storage (i.e. SSD, SSHD), add RAM, Maybe upgrade the CPU (if one was available for that model).
But replacing things like the motherboard for newer tech, its not really possible. The case is designed for a specific motherboard layout and finding a new one with not only the right layout, but the right external ports would be extremely difficult. This is why I tell my clients that when they buy a laptop, get a good one as when it comes to most parts, the laptop will always be as they order it.