The idea with ATV and purchasing movies is not to store it locally at all.. but to download it on demand.. your purchase is recorded and you will have access to the movie at any time.
To store movies it will be loaded in your local itunes library..
You cannot store downloaded material outside of the itunes library.. AFAIK.. so therein is your problem. If you move the itunes library off your laptop then you can use external drive plugged into Macbook.. that is the best solution from speed and reliability point of view because TM can then back it up. TM cannot backup network drives.
I am unsure what product I should buy? the Time Capsule or the Airport Extreme?
They are the same.. except TC has internal hard disk and is designed for TM backups.
Extreme has only USB port. . exactly the same as the TC.. but doing backups via TM to the USB is not reliable.
Your USB disk will be extremely slow.. and not recommended on either AEBS or TC.