Wouldn't it make more sense to start with the JAR writtens, and go on from there? For that you just need a PPL.
Depends on your timeline really. LASORS (copy on the CAA website) has all the requirements for converting plus the rules governing the JAA writtens. In terms of the writtens though you have 18 months from the date of your first pass to get all 14. Six sittings in total to do them all and 3 attempts at each paper ( I think). Then once all 14 are passed you have 3 years to get your JAA IR or else your thereoretical credits are lost again. I think that is it in a nutshell but you had better double check to make sure.