[jononor]

Hardware (and sourcing) is harder than software. So I would heavily weight which OS is supported for the kind of devices you need, from reliable manufacturers with long support times. Always have multiple sources for the type of hardware used.

You also need to consider who is responsible for monitoring, maintaining and updating the devices OS and software, and whether it is a pull or push type update model. If it is you, then the fact that customers IT department knows Windows is not a benefit (on the contrary, they might cause problems).

When it comes to language and GUI framework, pick based on availability of skills. Having people that know X is more important than what X is. I would try to find a way to solve the modification issue which does not limit your choice of tech. Not being able to use JavaScript/web for UI is a huge limitation wrt access to skills. Tamper protect/detection using cryptography.