I'm 2 for 2 personally and 3 for 3 on family recommendations buying refurbished Dell Latitudes. I currently have the E6430u which is very nice and light with good Linux support; that's a bit old now but I just checked and you can get an E7250 (i5, 256GB SSD, 8GB RAM, HD Screen) for under $600! And you can upgrade the memory... yourself!

Just make sure you double check the wireless card support on Linux. The Latitudes use a variety of different chips.