I had a 320GB SATA drive from a laptop I recently upgraded to an SSD, and wanted to put the 320GB drive in the Mac Mini as a data drive.
The install went extremely well. The dual drive kit I purchased from iFixit had all of the parts and tools needed for a successful install. I followed their online guide for installing the drive, and all went smoothly.
I strongly recommend having a large clean work surface where you can spread out and organize the parts as you disassemble the Mac Mini. This is true with any piece of hardware your working on. The better you organize your disassembled parts, the easier it will be to reassemble. I've been custom building PCs for many years. I do have to say, the Mac Mini is a well designed machine. Easy to disassemble and reassemble. I'm well pleased with the results of the install, and the performance of the machine.