18G.
How do you convert to a single seat D9? Just the same as you would to any other single seater. You get training on a similar two seater, (I'm not that familiar with Jodels but there must be a small, underpowered two seater somewhere in the line,) then you have a very good briefing from a pilot experienced on type and finally you fly it, for several hours, exploring towards the edges of the envelope, from a big field in good conditions. You do not, ideally, have your first trip from a short field into another short field in someone elses aircraft, no matter how much of a hotshot pilot you are.
Mike W