I think the best way to frame this is that the desire for autarky and Lebensraum were both informed by the Nazis' ethnic Social Darwinism, with certain caveats and subtleties; to a degree, the desire for autarky was pragmatic. Germany had a more or less permanent foreign exchange shortage, which made the imports their industries and agriculture depended on scarce and expensive. However, the desire for Lebensraum was ideologically and politically motivated (as much as 18% of the German population could be classed as peasant farmers living off plots of land much smaller than optimum for contemporary farming techniques) and was obviously closely related to the Nazis' views of the German race being in a state of existential competition with the Slavs.

For a fuller treatment of the Nazi economy and how it tied in to their ideology I recommend Tooze's Wages of Destruction, which along with Paxton's classic Anatomy of Fascism are my main sources for this post.