I picked up an Ultimaker 2+ with 15,000 hours on it at an ewaste junk shop for $100. Really was expecting it to need some TLC, but I plugged it in, leveled the bed, and it spit out an absolutely passable benchy in a couple hours. Cura (Ultimaker's slicer) isn't open source, but it is free, fairly unobtrusive, and works fine. The printer itself isn't connected to anything but mains power, and loads gcode from an SD card.