Nehru didnt start off as a socialist. He was a soft core Fabian socialist influenced by Britain. It was in the later stages (post independence) that Nehru became very influenced by the success of Soviet Union and wanted to emulate some of their policies. But even then he vehemently opposed to the Communist party of India. they were his major opposition.
Why Gandhi picked him? Nehru was young a charismatic leader who was very influenced by Gandhian policy and he had immense popularity. Lets understand one thing, Gandhi and Nehru were not opposed to socialism but at the same time they didn't have an absolute ideological affinity to it. Their first objective was to free India, prevent chaos-disintegration, build stable government and political system. Socialism or capitalism wasn't the first thing on their minds. Nehru and Gandhi had several ideological disagreements, in fact towards the end they actually had a fallout. Nehru was an ardent believer of building of a modern westernized industrial economy. Gandhi was opposed to industrialization and wanted to focus efforts on building rural economy. India's economic success/failures basically epitomizes the struggle between these two opposing view points.
I don’t think Gandhi ever picked Nehru to be his successor in the decisive sense-he in fact wanted to disband the Congress party on Independence. Gandhi encouraged a broad field of leaders of all hues-including C Rajagopalachari (free-market believer), Sardar Patel (right-leaning conservative) and of course the left-leaning Nehru. Patel and Nehru were in a sense, like your nickname-Yin and Yang.
The tragedy was Patel’s death in 1950 left Nehru without a capable rival who had similar popular backing and administrative and political ability.
You are wrong about Nehru. He was ideologically a Marxist and he himself has said so.