I would start with Americans with Disabilities Act (ADA) Standards for Accessible Design, here: https://www.ada.gov/regs2010/2010ADAStandards/2010ADAstandards.htm
This is what the USA is required to use when designing public spaces to ensure that it meets the minimum needs of most people living with disabilities. Personally, I feel it does a great job in terms of accessibility. If you’re not from the USA, then you may need to convert the units from inches.
I’ll reference a couple sections by number from the link mentioned above (to quickly search the document, open the link, press Ctrl+F, a search/find bar should appear at the bottom, type in the code #):
- 306.2 Toe Clearance. This should provide some details for clearance under the machine, ideally all of it should have such clearance, but at least the touch screen and pay keypad area should be clear if you want most people in wheelchairs to be able to use. I would also try to do more than 9-inches for the toe space height, as power chairs often sit higher (mine personally is adjustable angle, so I could fit the minimum, but would need to lower my angle all the way down).
- 308.2.2 Obstructed High Reach. This is a good example of reaching over a countertop, personally I do not have this much reach and would rather see things at or near the closest edge of the countertop (or even built-into the countertop edge).
- If I must sign for my credit card, I would prefer being able to bring the signing pad to me (I have a tray attached to my chair, so I would set it on my tray to sign).
- Vocal commands could be very useful, assuming the ambient noise levels are not to high for the unit (machine: “Are your items correct?” me: “No,” machine: “Do you need to add or remove an item from the list?” me: “Add,” etc…).
- It may sort of defeat the purpose of self-checkout, but I would also like to have a “call for assistance” easily button available anyway. This could be useful for people who can manage the checkout, but need help getting the groceries into their car.
That’s all I can think of off the top of my mind, I hope it helps. Nevertheless, if you have any additional questions, feel free to ask.