Well,,, that oven has been to the moon and back a number of times. As long as it is still working, it is fine.The oven is a regular electric, and about 45 years old.
My suggestion,,, Is keep a notation of oven setting temp, and actual temp. This way next time you decide to bake breads, Set it at 325°F and your resulting oven temp will be 400+
When baking cakes, set at 275°F and resulting oven temp will be about 350+
I have a feeling that the $20 thermometer is close enough in accuracy.