Any of the shared hosting plans should be able to handle this level of traffic easily, so you might as well save yourself some money and go with the level 1 plan.
There are quite a few ways you can achieve what you want. One option would be to create a new machine user and sub-domain for each of your 'customers' and provide them with the FTP log-in details for these, they could then upload the files using an FTP client. There are a few 'issues' with doing it this way, mainly to do with file permissions, but they can be overcome with a little extra configuration.
Probably a better option would be to obtain a script to handle the uploads, this way your 'customers' would be able to upload their files using nothing more than their web browsers. There are many such scripts that are freely available and most will work fine here at DreamHost. A good place to look for such a script is...
There is one issue that you might run into with this second option. If the script is written in PHP, then it will be limited to ~7MByte for the uploads, obviously this is not adequate for your needs. Thankfully, this PHP limitation can be overcome by installing a local copy of the PHP executable and PHP.ini file in your domain, but this procedure is a little involved and if you are not familiar with the SHELL, you might have difficulty.
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