There's no real benefit to using an agent, unless there are special deals from Colombia - and there may be.
It's a bit early for discounts for January, but monitor mousesavers.com and check the disney website regularly for specials. It's the least expensive time to go. Even with that, it's still not cheap
IF they come the first two weeks of Jan they will most likely have the place to themselves. It will be busy for MLK weekend, but other than that, it will be empty. FWIW - I'll be there the end of Jan