I have set up a Raspberry Pi and it was a great way to learn to install an OS, LAMP stack and Moodle from scratch. It was perfectly functional and with a tiny router powered by a USB Powerbank, made a portable solution I could just about carry in my pocket. I loved being able to set it up, then wipe it and start all over. I could see it being used where there was no power, no internet but the students had mobile phones.
But a PI is really an under-powered beast with 1 gig of RAM, a not very powerful CPU and uses a SD card for writes which has a limited life. With the number of students, something a bit more powerful would be required and these days many obsolete and discarded computers would have far better specs. Cheapest solution might even be a laptop with a dud screen.
I think unless extreme portability is essential for a small number of students, a Moopi is not a practical production server.
As for the rural school - my advice is suck it and see. The server is a good start and is what you have. Set it up with a LAMP stack (if you can - Windows is not considered optimal for Moodle) , install Moodle and get started - perhaps with one course at first. Don't underestimate the time required to create course material and for teachers to become familiar with the system. I think the best way to learn Moodle is from running it.
But I do wonder - when will the students use the system? If it is independent of the internet and based around a server and wifi, how will they access it without being in wifi range?