I agree with Kaz as well.
My guess is that you could get a GFA instructor to train you in your RAA reg glider, but see comment about Form 2 below.
My understanding is that the altitude limitation comes from the pilot's license, not the aircrafts license. So, if you had a suitably endorsed GFA license and you were flying your RAA registered glider then you would be flying under GFA altitude restrictions.
There are no altitude restrictions for a GFA pilot but airspace rules do apply. For most of Australian airspace this means that the pilot must get a clearance before going above FL200 (20000 feet). I think that in the near future this clearance is going to require ADS-B Mode S.
I will speak to my friend who has the Sinus. As I have said my understanding is that he has registered it with the RAA but uses a GFA pilot license with Power and Independant Operator endorements. I will post here what I learn from him. It may take a few months though because he and three others are flying their motor gliders from here (WA) to Bourke Town, QLD for some Morning Glory fun. I may not see him before he leaves in September so it may take a few months.
I think that the only practical difference between registering a Taurus with a VH registration and a RAA registration is that with the former there is a requirement to do a Form 2 on the aircraft every year. This is a good thing! All the motor gliders, other than the one Sinus, have VH registration.
I think that in your position I would take the following path:
Join a gliding club and get to the "Silver C" stage.
Buy the Taurus.
From the same gliding club get your Power and Independent Operator endorsements, and Form 2 ticket.
Go flying in wave.
Good luck.