Given the parameters of the choices (e.g. 15 files at 400MB each), it appears there will be an impressive 6 GB of content, roughly. At 300 kbps from the LLVJ server, that's about 44 hours to download it all. A long time to expect no hitches in the network connection.
I got close to 600 kbps for the 2nd part of the Antarctica segment (a few days after the initial surge died down), just under an hour for the roughly 250 MB file. That's still about 22 hours for the whole 6 GB.
I suggest that you size the base files to take at most 1.5 - 2 hours to download at the maximum speed you think your server can reasonably attain per connection. Then even if someone gets only half that, a single file can still be downloaded in an evening. Adding some zip containers with multiple segments for those who want them shouldn't be hard.
I've found it fairly easy to track what I've downloaded through the ZenCart interface, so far, though once the number of files gets into the serious double-digits, it would be harder.