Ok, then for something being accessed via a webpage then you could run that off of a gen 3 raspberry pi easily. You will then just need to install a virtual keyboard program on it.
So you are talking $150-200 for the 27" monitor, $35 for the pi, $15 for a case and power supply, and $10 for the micro SD card.
The pi runs a specific build of linux, it is really easy to setup but you can always buy an SD card preloaded with Raspbian on it as well.
I could not find any monitor AND raspbery pi VESA mounts but you can easily take a piece or two of 1/8" thick metal and cutout a bracket to connect the wall VESA mount to the pi.