make your own low cost PC based on ITX or entry-level all-in-one ATX motherboards) without screen. You can use a KMV (keyboard/mouse/video) switch to use your existing PC's peripherals to access the system when needed.
I like this idea. How do I use a KMV switch? Is this a hardware or software?

The file as of the moment is just around 20mb, for around 2 months of data. I don't know how big this will get after a few years of data.