Since you don't live in Southern CA, you only have two choices. The Deluxe and the Premium. Right now, as far as I can remember, the Deluxe is 259 and the Premium is 379. Or maybe it's 279 and 359? You can check on
disneyland.com.
When you upgrade, you pay the difference between your ticket and the price of the AP. The AP gets a start date of the first day you used the ticket that you upgraded. So when we went in September, our first day was 9/18. We upgraded on the 20th, but our APs were dated to the 18th, meaning that is when our AP year started.
You go to the bank inside DL (go to the right through the tunnel and you'll see it past the sign to the bathrooms) and they will get it taken care of, including the picture.
If you'll be using the Early Entry from the ticket, do that first, and then upgrade. If you're staying onsite and get EE from the hotel, then you can upgrade whenever. Once you upgrade, you get the discounts! A deluxe gives you 10% off most food. A premium gets you 15% off most food, and 10% off most merchandise. They give you a list of restaurants and such where you can use it when you upgrade.
Be sure to find out when the blockout dates for the Deluxe are, to make sure those dates work for you. If you usually come during blocked out times and you can't change that, then you woudln't want to get a Deluxe, and would want the king of the APs, the Premium, which gives you 365 days (though I suppose with the leap year coming up it's 366, isn't it?). Oh, I re-read, and you said you visit during slow times, so the deluxe should be fine.
have fun!!!