You can definitely grow bush-type squash varieties in pots; the vining types may get too unwieldy grown that way.
As far as longevity of pumpkin seeds - buying them now (& probably at discount prices) for sowing next year will work just fine, as if stored properly the seeds can remain viable for years. I've had the seeds from rotten Halloween pumpkins sprout the following spring all on their own after having endured the weather & frigid temps of winter. I've also had pumpkin seeds sprout in the yard after having - ahem - worked their way through the digestive system of a dog. Now that's some diligent germination - lol!