The speed limits in Turkey are 50 km/h (50 mph) in built up areas, 90 km/h (56 mph) outside built up areas, and 120 km/h (74 mph) on motorways.
In Turkey the motorways are numbered and prefixed with the letter D, the non motorway main roads are numbered and also prefixed with the letter D, the smaller roads just have the names of the town that they lead to on the signs.
The Turkish motorways and the non motorway main roads that I rode on are of a good standard, however around the Istanbul area expect extremely heavy traffic at all times. The condition of the smaller roads are generally good but there is always the odd exception.
The motorways signs have a Green background with Orange/Black numbers for the motorway number, the non motorway main road signs have a Blue background with White numbers, and signs for the smaller roads have a White background with Black names.
Turkey are not in the EU so they still have their national borders and depending on what insurance policy you have for your bike, you may be required to purchase insurance for it at the border. As they also do not use the Euro in Turkey and petrol at the time of writing was the equivalent of about 0.65 Euros per litre.
I saw any signs for average speed cameras on the motorway but I never saw any patrolling police vehicles.
Turkey has lots of great biking roads but my experience of them is limited to the West of the country and all of the roads that I rode on were very good for bikes.
To buy a SIM card for a mobile phone you need to go to a mobile phone service provider shop and you are required to produce your passport to buy the SIM card. When I did this the cost was around 10 Euros for one month of internet.