Navigation Data
Up-to-date navigation data and charts are a must wherever you fly. This includes data onboard aircraft avionics and electronic flight bags. Between Jeppesen and ForeFlight, you can obtain the necessary coverage; substitute Garmin as required.
Aircraft Equipment
For a Part 91 Atlantic crossing to Europe, at a bare minimum, you need one long-range navigation unit (LRNU, aka GPS), short-range navigation equipment (VOR, DME, ADF), VHF radio with 8.33 spacing and immunity, a Mode S transponder and 1090 ES ADS-B out installed in the aircraft. An altimeter capable of toggling from inHg to hPa is not required but would be helpful.
ICAO Equipment Codes
A Part 91 operation equipped with WAAS GPS, ILS, VOR, VHF Comms, ADS-B Out, and no LOAs will likely have the following equipment codes (ForeFlight Format)…
Survival Equipment
The US, Canada, Greenland, Iceland, and Europe require a long list of survival equipment for single-engine aircraft. Beyond regulation, consider Fire Containment Bags and survival training courses.