I think there are three tiers for determining what Nebraska can expect based on recruiting rankings (these should be viewed as rough standards rather than exact requirements):
Top 25+ recruiting: compete in west, occasionally win the division, lower percentage chance at winning B1G
Top 15-24 recruiting: compete in west, regularly win division, moderate-to-good chance at winning B1G
Top 5-14 recruiting: win the west, good chance to win B1G, CFP contender
So far, Clemson has the worst average recruiting ranking over the last five years of any national champion (11.8). All of the others are in the Top 10. I don't think Nebraska is going to sniff hopes at a national title unless they get somewhere in that range.
To the question at hand, 'success' definitely does not require a top 5 class here. Solid recruiting in the 15-25 range will position the team well for conference titles, major bowls and potential CFP appearances (the latter being unlikely unless they can get into the top 15 recruiting realm).