If it works it is definately a bargain!! If it doesn't work, based on ebay prices, it is still worth more than you paid!! Having the box will also add to the value.
The crystal doesn't look to deeply scratched and there are products (eg Novus) available to buy which will buff it back to almost like new condition and remove the 'dark circle' you refer to.
The batteries are cheap and easily obtainable so it definately worth a try, if the old batteries have been removed your odds of having a working watch are improved as leaking old batteries are a big cause of irreversable module damage, the batteries you will need are probably LR44 - A13 USA size..... I think.
This site is a good source of information if you have any problems, especially for beginners (like me)
Welcome to the forum and good luck.