Is Sept a good month to visit Istanbul?

I'm think'n' about going to Istanbul for a week. Is that enough time or too long to spend there. Will my money go far? I was thinking about bringing just as few clothes as possible and just picking up things there to wear. Any suggestions? I've never traveled outside the country before, do I need anything other than my pport? What things are there to do? Is there nice beaches? Is a cheap hotel okey? That way I can spend my money on other things. I'm no youngster so I'm not looking for too wild of a time. Any experiences there you would like to share? Is there some where, maybe, better to go?