I get bikes here in Cambodia that came from Japan in a container, they are anywhere from $40 to $60 and probably would be one of the best bikes that LBS should provide.
Here is one of them.

1x7 speed and it just works. Cheap build but it works

Here is one like above I stripped and made into a coaster brake only bike.
Besides most people do not understand the idea of the front shifter therefore the 80s mt bike is not optimum, make it a 1x and get rid of the triple chainring would be simpler and easier to work on.