You must have the theory examinations passed before the flight courses.
As you are looking to obtain a JAA CPL/IR ME then you will need the JAA ATPL exams passed (or JAA CPL and IR exams) before you start the courses.
The only requirement to enter the exam room is to be a holder of an ICAO PPL.
A JAR 25 aircraft is otherwise known as a Performance Class "A" aircraft. It covers the regulations required for Jet aircraft, no matter the size and Multi Engine Turbo Prop aeroplanes with maximum seating of more than 9 passengers or greater than 5700 kg MTOM.
JAR23 covers piston powered aeroplanes and turbo prop aeroplanes with 9 seats or less and a weight less than 5700 kgs.
They are similar to the FAR23/FAR25 regulations in the USA.