doodleb' - I can't help with Iseki v J Deere and neither am I greatly experienced in tractor matters: not much help then, but I'm posting to ask whether you are considering new, nearly-new or "it's done quite a few hours". Also, are you looking at hydro, shuttle drives etc or just plain ol' mechanical gear boxes etc etc.
I'm thinking from your post that you are thinking about a new one, but it might help other members decide whether they can help or not if you could fill-in a few details of the models you might be considering. Just a thought!