An unit that gives you voice directions is perhaps the best idea, however you may would like to figure out whether you can switch the speech off before you buy.
Consider quality-- you only want to have to buy and install this once, and you do not want it to break down halfway through a very long journey. Buy the very best you can afford.
Not all of the in-car GPS devices offer real time tracking, where the device is frequently updated on your position-- ensure you get a system that does provide this.
In the long run you need an equipment which you find easy and simple to use. Otherwise you'll never use it and all the money you paid on it will have been lost. This piece of modern technology may be a blessing in those dark empty road in the middle of nowhere situations, but see to it that you do not end up with an in-car GPS device that fills you with fear just at the consideration of switching it on.