Hello Xauciweez88
Supersaver tickets are tied to the connection chosen upon ticket purchase. We do not recommend purchasing Supersaver tickets when arriving by air; they become invalid once you missed the booked train which is a likely scenario with air travel involved.
Rather look for a Saver Day Pass valid on any train/connection.
Online tickets are good for travel, no exchange needed.
Whether 1st class is better? Yes for a 3hrs travel following an international flight.
Large suitcases can become an issue. Everyone is nowadays travelling with XXL bags. You must not block access doors or emergency equipment with your baggage.
Kind regards,
Roland