Mr. Fox I feel bad that this is all turned out this way for you as you've entered the hobby I do think it'll turn out for the best. Very unusual to get two tanks like this back to back.
I wasn't aware that you picked up a panther until I saw this latest part of the thread. I just sold off a big part of my collection to make room for some new builds and some beautiful tanks I picked up from ( Michael C )on the Forum. I've just sold a few Tigers and a Panther.
If anything you can chalk this up to learning experience it would be good to find out what exactly was ailing both of your tanks so that you and others will know in the future if something like this happens
A few years back I had a taigen Panzer III everytime time I would fire the main gun the engine would cut off and shutdown so after driving myself crazy for a couple of days I switched everything over to Heng long Electronics and it worked perfectly..... So in theory I had a dodgy mainboard that was causing this thing to shut down.
Now that was a primarily customized tank so it wasn't brand new out of the box and your case you're going to have to resolve the King Tiger first but it sounds like you might have something going on with the Taigen board in the panther
Best of luck to you
Carl