Do not hurry to buy new or even used navi unit.... Try to reanimate the existing one. It is quite difficult to kill MK4 (but fo course still possible ;-) )

Download the navi SW, burn it on the disk. Before the update make the reset of your navi (close all doors, open trunk, wait till the light on the navi switched off, disconnect 2 jacks from the behind, wait for 15-20 min and connect navi again). Then try to update your unit with the burned SW. It could take some time when your navi starts to update.

Good luck!