Old houses perform poorly even when insulated after an extensive renovation. This is more to do with the method of how houses were built vs to today's slab concrete foundation. While insulation will help, IMO I find draft proofing being the bigger problem of NZ houses. DVS HRV ventilation system or not, if the house is not built with tightness in mind, then the operating cost for ventilation will be costly compared to a modern new home.
Your mileage will go further if you pick a house where the windows face mostly to the north for maximum sun exposure. Though windows are the weakest part of the house, there's a huge cost in re-trofitting newer double pane windows to older houses and the end result will still never be as good as a newly built house today.