Ordinarily I'd say you're nuts for planing such an ambitious cook your first time out, but the Smokin' It is about as close to an oven as you're going to get. Main concern is going to be keeping smoke rolling. It's easy to overdo the smoke on those, as most folks feel like they have to reload the box throughout the entire cook. However, I believe 2 or 3 loads should be sufficient. Another option is a dust or pellet smoker like the Amaze-N-Smoker line. Not sure how well they burn in the Smokin' It smokers, as that drain hole won't provide a lot of airflow, but hopefully someone with more knowledge than I will come along and give some input on that. I seem to remember reading someone either enlarged the bottom drain hole or drilled additional holes, but I could be wrong.
I'd do the briskets and butts the day before and reheat, but that's just me. I'd think 20 hours should be enough time, but only just. If you're going to foil, you might save a few hours. Make sure you have a way to monitor temps and you're familiar with probing for tenderness. A bit of reading through the posts here will get you in the right direction.
Good luck, have fun and come back and let us know how it turned out!