Can I use MCP3008 to input analog signals to Raspberry Pi Zero? If not, Which is the cheapest version of Raspberry Pi?
Yes, you can. There's no difference in the GPIO/SPI capabilities of any Pi models.
Here's a guide which includes wiring, enabling hardware SPI and reading analog input signals with Python (though you can use anything - this is just the simplest way): https://projects.raspberrypi.org/en/projects/physical-computing/15
Also see the gpiozero docs on MCP3008: https://gpiozero.readthedocs.io/en/stable/api_spi.html#gpiozero.MCP3008