Although I doubt it would have had 2/3 doors.
Happy to be corrected but I would have thought the 156/158 end of coach doors would have continued.
The only artists impression produced contemporary with the proposal shows a bodyshell similar to that of a 323, with the leading door at the one third position, as seen on the Wikipedia article and described above:
https://en.m.wikipedia.org/wiki/British_Rail_Class_157
It doesn't seem outlandish to assume that Hunslet might have used an adaptation of the 23 metre class 323 bodyshell, that they were already producing, for such a small order.
Regarding the potential number range for these units, 1571xx, 1578xx or 1579xx would have been most likely as ScotRail used the last 3 digits to identify DMUs.
0xx - Class 107
1xx - not used
2xx - Class 150/2
3xx - unrefurbished Class 101
4xx & 5xx - Class 156
6xx - Refurbished Class 101
7xx - Class 158
8xx & 9xx not used.
Every written and online source I've seen has stated that they would have been numbered in the 1576xx series (The artists impression linked to above carries the number 157001, but I don't treat this as gospel). As noted above by
43096, the numbering sequence for 15x series Sprinter units was sequential, so 157s would have occupied the unused 600 series between classes 156 and 158.
Note also that, as mentioned earlier in this thread, if the 157s had been built there probably wouldn't have been 101s refurbished in their stead, so the 6xx series would have been available anyway.