You're going on Thanksgiving weekend! The 25th will be "not-so-busy" (average of 30 minute wait or less), but the 26th will be ultrabusy (45-60+ minute average).
Weather is usually great (70+).
As for getting the express plus online, I believe it is possible, but I foget where they can be ordered, if at all. Predominantly they can only be gotten at the park(s). And yes, the express plus can run out.
The free express can also run out and WILL run out on popular attractions (Mummy, Spider-Man, Hulk, Shrek) on the 26th.
And as damo said, you would probably be better off trying to stay onsite. Each person needs their own express plus pass each day, so the cost can add up to a night at the RPR. And the "5 at US/5 at IOA" no longer exists. They had that express as part of a Kodak deal, and those tickets expire at the end of this month. Express plus is park specific, so you'd be paying the extra $25/person both days...