I don't know how good sat nav is that is already pre installed in cars but my dad has it in his Mercedes (the computer thingy) and he said it worked perfectly until they got into Turkey itself. I think that is the main problem with most sat navs - the Turkey part of the trip.
We had a Garmin sat nav when in the UK and we could get it to work for all of Europe except of course Turkey - you could buy a programme for it but it was a ridiculous amount of money, if I remember correctly more than the sat nav cost!!
If you go to the RAC website you can put in your departure point and your destination in Turkey and it will give you a route.
If you have already a Sat Nav and it takes the standard SD type card you can buy justthe updated card for all of Europe including counties such as Turkey from Amazon and Ebay
I know bot tom tom and Navman cards were available on both and I got the navman version which actually changes the software whilst its in to I-Go i think but as soon as i swap cards back to original Navman it works as it did before as Navman sat nav software.
My card was only £19 from ebay and i think the tom tom one from Amazon was only around £26 if i recall correctly from the poster that 1st got me on the trail of getting mine back in May / June this year.