PCAP touchscreen worked well with double-gloved fingers and there are touch controllers that will reject a stray water droplet on the screen.
I'm not sure about the OS considerations for medical device use, but for linux you might consider the yocto project, which is designed for this kind of stripped down deployment. It will limit the number of moving parts you have to validate. If you're less devoted to the open source route, check out QT's boot-to-qt offering. QT ui's are common in this field and I believe there are supported boards at a range of price points.
You wouldn't want to hitch your design to Rasperry Pi or Beaglebone or such because of the short support lifetime of individual models and components. Part of what you pay for with the industrial vendors is a commitment to supply the system for 5-10 years.
If any of this is helpful, drop a note using the email in my profile.