Can’t rely on Wayback Machine to ensure images don’t get lost - even less than a paid 3rd party service. Hosting our own is not expensive, and I argue should be MVP.
One way to keep the cost down initially, and would be simpler as well, is to not store any images that are retrieved as part of transferred Q&A from SE (or other sites) and link those images to the original location (imgur or direct hosting of any other site). That is quite logical and eliminates the initial burden of transferring (potentially) millions of images. It also may avoid some licensing issues.
I do agree that built-in editor is not MVP. But I believe that the 3 key functions (crop, rotate, resize) can be done fairly easily and would be an example of an advantage over some competing systems.