If you were my client and the existing HWC is in the roof I would probably recommend putting the new gas unit there as well, mainly to keep the existing hot water layout. Sometimes when you relocate your up to all sorts of surprises. I guess if its your place you deal to it. Downturn of roof space is that the unit is more $$$ and not easy access to trouble shoot or service. Depending on your unit not to sure about freezing, I live in Auckland so never had an issue, maybe other guys further south might know.