Data migration from old org to NPSP

We're currently using an old version of Salesforce which was specifically developed by "Give Clarity" called "Supporter 360". We also have an instance of NPSP which was initially set up by a predecessor who started creating fields ready to migrate. There would be around 5000 contacts to migrate, but also organisations, batches, campaigns, projects, reports, regular giving. We are a very small charity without much of a budget- I'm already working on identifying unnecessary fields etc. and am willing to put in the time, but only have basic Salesforce skills.