I'm (still) in the process of re-purposing an old laptop for a home server. 2TB laptop hard drives are a reasonable price, and on older laptops with a CD/DVD drive bay, you can usually get an adapter caddy to put in a second drive. Pay attention to the allowable drive height though.

This system is going to be set up with software RAID.

I expect to have higher compute power, more RAM, and much better IO throughput than a RPi solution. Also, the laptop battery (which is still good) serves as a built-in UPS.