Note that really high temperatures will affect the retention time of EEPROM and flash memory. If the PSU has a fan, I would recommend that the input air can flow over the processor. I have seen several references that every 10C is a factor two in retention time. Looking for 105C parts will greatly affect the availability. Maybe you should also discuss your application with the chip manufacturers application engineers when you are getting ready to choosee a chip.

The Keil tools may be for Windows. But the USB examples should be possible to port to gcc if you just find a suitable startup file to begin with. Before selecting a processor, you should spend quite some time looking at what code examples you can find for it. Especially for the USB part.

Supporting firmware updates over USB is important to add as a requirement when selecting a processor. And you should directly start thinking about a boot loader strategy. USB communication in the boot loader or storing two full applications that handles download of data for the alternative application and then requests the boot loader to start the other application instead. With a boot loader, you might want to take advantage of the locking strategies the processor supports to protect the flash contents.

I wasn't talking about the transfer speed of the I2C, but how fast the ADC must sample data. You may have a need to run the ADC at a way higher speed than you can transfer - for example to perform some filtering and analysis of the data. This may also affect your CPU needs.

A resolution of 8 bits gives a maximum precision of about 60mA. But the last bit can't really be trusted because of truncation, offset errors, ... So you may have a measurement error of maybe 100-150mA. If you switch to a 10-bit ADC, you will get a higher resolution. But that does not mean that you get a higher precision. A PSU working with high currents may have quite high noise levels, that will affect the real precision. In the end, you might end up with maybe +/-50mA of precision - but that also requires that you care about your voltage reference.