New to CiviCRM: Hosting (VPS) requirements and migration from NationBuilder
Q2 JMA Consulting and Fuzion recently migrated a large db from NB to CiviCRM + Drupal. JMA built the migration mapping. Fuzion worked on a Drupal-side interface that replicated some of the common NB screens (so not relevant for WP sites). You may want to get in touch with JMA via the Partners list concerning the migration side of things.

NOTE: If this is seen as too 'advertorial' then pls down vote it and I will happily remove it.