We can offer additional storage on s2survey.net, but as the storage is secured by a multi-layered backup (usually intended for uploaded material from respondents), it comes at a price of 49 € plus tax per additional GB (available for max. 1 year). It may be possible to have 2 GB for 49 € if you need the storage for less than 6 months.
Another option is to store the files on a university server and simply embed them via HTTPS URL.
To use s2survey.net you will need a commercial licence. How many respondents will "minimum population sample of 500" likely be?