I just had a look at your profile and it looks like the back problems are quite serious so I'd definitely get test rides before committing.

I would also seriously look at tadpoles with rear suspension (such as the Trice Q/T or HP Velotechnic Scorpion) or a long wheelbase delta like the Greenspeed Anura.

I found the Anura to be a very smooth ride, even over bumps that I flinch from on the GT5, but not as stable for high speed cornering.

Getting tests on those tadpoles could be tricky but Greenspeed are in Melbourne...