Hi Wolfgang,
If by Old World breads, you mean those great German ryes I want to know what you know!
OK, for potato doughnuts, here's a recipe I got from an Amish baker, but never tried it.
1 cup sugar
1 cup mashed potatoes
1/2 cup lard or shortening
3 eggs beaten
1-1/2 tsp salt
1-1/2 - 2 packages yeast
1 cup warm water
5 cups flour (altered as necessary depending on your flour).
On this recipe, you might also want to change the water to scalded milk, as in the other recipe given here.
Now, since you mentioned you're a diabetic, here's a link to a site which is a very special bread site, for Baked donuts, not fried, if that helps any.
Yeasted Baked Doughnuts Recipe | Wild Yeast
Now there's also cake donuts and yeast donuts, I'm assuming you're after yeast donuts. but maybe you're after a cake donut, they're always tender, but generally more dense and not as light as a good yeast donut.