Where to start indeed! I would start by perusing the DS2 section of this forum, as well as the more general categories. Use the instructions here if you want to search for specific keywords:
viewtopic.php?f=41&t=3439My DS2 was a '79, so most likely very similar to yours. I have lots of pics in my gallery of various fixes/improvements I made.
The next thing you will want to do if you are going to convert to the religion of DSism is learn how to pray to Saint Rudy of Assonet. Well, OK, not actually pray, just call him! Or if in the area stop by. Rudy used to work for O'Day and knows these boats inside and out. He now owns D&R Marine in Assonet MA, and stocks and sells boat stuff, especially O'Day-specific boat stuff. Go here:
http://www.drmarine.com/categories.asp?cat=64As to your specific bullet points:
You can easily get replacement inspection ports from D&R or many other suppliers.
Once you get them open or replaced, you can plug the drains, then run some water into the bilge. If you see any leaking out, you'll know how it's getting in. If you don't, it's coming from another source (rainwater?).
The cable that is shot is the centerboard uphaul, and can easily be replaced. D&R has ready made up replacements, or you could make up your own. I believe at least one forum member made one up out of Amsteel or the like.
Most of the SS hardware on the boat is "generic" and can be replaced/upgraded from numerous suppliers, but some is DS-specific and can be had from D&R (are you starting to see a trend here?).
Lastly, do yourself a favor and get some new sails for her, especially if you are just learning. Trying to learn with old blown out sails will be frustrating at the least, and you'll want new ones eventually, so why not just make the investment up front?
Welcome, and good luck! I could go on and on, but I'm sure others will chime in with additional advice as well.