probably wrong forum. First, you should be sure that legal-wise everything is clear, because you can sell DVDs only once, but download them as often as you want, so the retail price may be different for such an application.
Technically speaking: DVD ISO use MPEG2 codec, which is outdated and could do with 1/10th to 1/5th when converted to a modern codec, but you’d lose menus etc. MKV as container format could probably mitigate that somewhat, but I don’t know of any tool that would do that out-of-the-box, and in any case would have to be done on a per-DVD basis, so probably quite a lot of work. Otherwise, uncompressed ISOs are 7GB each, so you’d have to factor this in for server capacity and traffic volume.
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