The climate is perfect on the Aegean and Mediterranean coasts then, as well as in Istanbul. From late October to early April, the beach scene more or less shuts down. There's little rain between May and October except along the Black Sea coast, but from about mid-June, the mosquitoes come out in plague proportions in some areas. Eastern Turkey should really be visited from late June to September, as snow may close roads and mountain passes in the colder months. UK and US citizens do need visas, obtainable in advance at a Turkish consulate, or upon entry to Turkey.
Turkey is a low-slung dollar burner.
With the value of the Turkish lira always sliding, it's best to change money every few days. Keep an eye on all the zeros on your bills - it's easy to mistake a 500,000 lira note for a 50,000 lira note. Banks and exchange offices are generally only open Monday to Friday - you may find it hard to convert your travellers' cheques on weekends. Keep some exchange receipts as you may need them to change liras back at the end of your stay. In cheaper restaurants it's not necessary to leave more than a few coins in the change plate.