You will be told, quite rightly, that you don't need a Residence Permit to purchase a motor vehicle in Turkey.
However, from 1st January 2010, foreigners need a Foreigner ID Number / Yabanci Kimlik Numarasi and to obtain one you must be in possession of, either a current Residence Permit or have held one in the past.
Depending on the province in which you apply, it can take anything from 24 hours to 10 weeks to obtain a Residence Permit. In some provinces your passport has to be surrendered during the application process.
The costs for a Residence Permit are £40 for the first month and £30 for each subsequent month upto 5 years. If you employ a "fixer" to do the running about for you that will naturally add further costs.
The good news is that road tax and insurance are relatively cheap in Turkey.
Depending upon the length of stay, you'll probably find it far more economical and hassle free to rent rather purchase.
You might find this website interesting Route 69