Help With Building a Spider Bot

In the last couple of months I have been working with the Elegoo Basic Starter Kit[Mega2560]. I have completed all the tutorials.
Now I want to start learning to build a robot such as a spider-bot. I have watched quite a few videos related to a spider-bot.
I need help with what kind of parts I will need and where to buy them. I would like to build the robot from scratch rather than just assembling using a kit.

I am interested with building a spider bot that would be able to climb stairs.

Thanks.

Hexapod or Octopod?
Hobby servos for leg movement?

The PCA9685 makes it easy to connect many servos and distribute power to them.

Weight is an issue. just a few servos have to bear the weight of the entire robot.
Depending on the length of the leg segments, it is easy to exceed the torque capacity of the servo.

Step climbing presents additional challenges.
You need to be aware of where the step is so you can change from floor walking to step climbing.
You need quite long legs to reach up the 7.5" (19cm) human stair rise. The legs need to be long enough to let the robot lift itself up until the center of gravity is over the feet on the upper step so that the feet on the lower step can be raised.

Long legs require more torque, making it easier to exceed the torque of the servo.