This has currently been put on hold.
The SATA-->USB converter we were planning on using was actually from a 2.5" HDD enclosure, which has now become EOL. So we will need to source some other SATA-->USB option.
Main reason for the internal convert was to actually keep size down, those in-line cable converts are kinda bulky and we didn't want things handing out of the case. Looks like we might have to go down this route anyway now.
VESA mount is a brilliant idea, will work that into the design.
It'll just be easier to design the case as whole, doubt there'll be an easy option to integrate into an existing case.
Thanks for the input, much appreciated