Ryanair WILL hit you with overweight costs.

I honestly can't see why you'd want to do this to yourself. For next May, Lufthansa will sell you a return flight from FRA to ARN for £65 (Euro 100), Expedia has just shown me. All-in. With taxes. No check-in fee. No baggage fee. No excess luggage fee if you are above 15kg but below 23kg. No trip to Hahn. No bus ride from Skavsta. No-one trying to sell you lottery tickets on the plane. Allocated seating. Probably a free cup of coffee and a sandwich each way as well. Departing and arriving to proper airports which are not converted military airfields with minimal facilities. On an airline which will offer you compensation or rebook you if the flight is cancelled or delayed. BOOK THE LUFTHANSA FLIGHT!