It's a great time to go with the Lira doing so badly, everything will be quite cheap apart from hotels - they correct for the currency devaluation almost weekly (though prices are still not expensive. First tip - it will likely be getting cold at that time of year so make sure you take suitable clothing!
Secondly - make sure you get a metro card as the metro/trams make the whole town very accessible (and you can use it for the ferries).
I advocate staying on the 'European' side/old city as you're close to all the main sites and lots of cheap/decent places to stay. When I was there in July the Blue mosque was under renovations. My highlights are the Aya Sofya, The Suleymaniye (my favourite mosque) and Topkapi palace/museum.
However, the more 'fun' and hip places are slightly further away in Karakoy, Taksim, Besiktas and Kadikoy (on the Asian side). Especially in Besiktas and Kadikoy you'll find less tourists and a more representative idea of how the locals live. Have fun!