Preamp. Analog-to-Digital converter capable of running at a decent data rate. Processor fast enough to pull the data off that converter at a decent data rate and write it to flash memory. Ideally , fast enough to do data compression. Software to handle that, including managing the memory-based filesystem. Sufficient, fast enough, memory for program and for audio storage and for working space. Interface circuitry, power supply, mechanicals for mounting and control. If you already know how to design/build/program all of that, it's not complex. If you don't, it is. Not a beginner project, I think, nor even a beginner microcontroller project. And doing it yourself will almost certainly cost you several times what buying one off the shelf would.