My 100D has developed a terminal looking fault

I hope this is the right forum, it looked the most suitable.  I have a much loved Canon 100D, which is now almost exactly three years old.  I was just taking some practice shots in the garden, as I had a hummingbird hawkmoth visit last night and hoped it might appear again today.  I was just trying to see what might be the best arrangement (focal length, shutter speed, flash or not etc.)  to try and catch it, as it moves so rapidly.

I went to take a shot (I'd already taken half a dozen) and the mirror appeared to lift (i.e. by sound), then not much else and I got an Err message in the display.  The back screen was black.  I switched it off and on again, still same error message and no shot possible.  Took the battery out (was the original Canon one), put a fresh one in, display came back on, could focus and take one shot, then same Err and black screen.

It seems that if I take the battery out, I can sometimes get one or two shots, but if reviewing images in the screen, it flickers, then the colours go funky, stripes form, then it goes black and nothing works.  I've tried variously cleaning battery terminals, lens contacts and swapping lenses, just to eliminate anything obvious.

It seems pretty terminal and not something I'm likely to be able to fix myself, but I wondered if anyone has any ideas about what might be causing it and how easy or economic it might be to repair - I suspect a new camera, or replacement might make more sense now.

I treat my cameras with respect and they all look like new.  They don't get bashed or battered, so a bump is very unlikely.  To my shame however, it did befall one incident when about 6 months old - a cup of hot black coffee got dumped over the entire camera when a gust of wind lifted a picnic table and dumped the contents in the car boot where the camera was supposed to be resting out of danger.  I dried it as thoroughly and carefully as I could at the time and it has worked fine for 2.5 years since and taken another 3000 images, seemingly without incident, other than one button has a slightly scrunchy feel to it.

This of course, does mean, that the hummingburd hawkmoth is sure to appear again and put on a fabulous act for me tonight.  And I won't be able to prove it!