I do Melbourne - Sydney - Bathurst - Melbourne (or the other way around) on average three times a year. Also Melbourne-Adelaide twice a year and loads more. Melbourne-Brisbane most years. All this in the trusty Mini Van. Standard 1100 (well, 0.060" oversize) and 10" wheels. I love it.
About the only things I would change would be a more comfortable seat (the one I've got is out of some sort of Bongo van and is getting a bit old and saggy) and either a better cruising diff, or five-speed box, and possibly a bigger motor - although the 1100 copes quite well, even at 110km/h on many stretches.
Whatever seat you go for, I strongly recommend a recliner on a long trip.
Hydro would be smoother, but fit shockers on the front to stop getting sea-sick! Morris 1100 is a better ride (I've owned one and loved it) but doesn't have the cool fun-factor of the Mini.
Remember though, as you no-doubt already realise, and as others have already said, it is a 30+ year-old car so problems will arise, so be prepared. Good maintenance, a few essential spares and know how to fit them.
I've been driving Minis and Mokes on long trips for over 20 years, and I don't have a hunch in the middle of my back, or a twisted right foot (twisted brain perhaps.)
Oh, and air-con from Jap import would be nice in summer.
Cheers, Watto.
