It really does depend on the problem you're trying to solve. If it is dampness, then air the house out regularly and make sure you don't have a rising damp problem (ground polythene sheeting can help with that).
The thing with relying on open windows, is you have to get a cross draft through the house which normally means cool side to warm side with a straight path through. It can be tricky to get that and I think that is where the ventilation systems can help since you effectively pump air slowly into every room. For example, two bedrooms on our house face north. One has another room directly across the hall, so opening the windows in both rooms (south side and north side) causes a draft through those rooms. The other north facing room does not have a counterpart opposite it so it is impossible to get a draft through it without some sort of fan forced ventilation.
Double glazing is pretty pricey and has one potential downside that most people don't consider. The double glazed windows will let sun and therefore heat in just like a single glazed window, but are better at keeping it in. In some situations, this can be bad as the thermal gain due to the windows is the similar than the single pane but the loss is significantly less. At our place, we have to chill the house down on a sunny winters day as the thermal gain on the north side of the house is huge. It can be 7 degrees outside but high 20's/low 30's inside just due to the thermal gain that is then trapped inside the house!
You are almost always going to get best bang for buck installing more insulation. Double glazing only increases the R value by a fraction, and you will still get a huge amount of heat loss out the glass. For ventilation, open a window, far cheaper. One of the big problems in NZ with mould etc, is due to people not opening their windows. They cause all this moisture to build up, but then seal their houses with the doors and windows all closed and not vents. Natural stack ventilation is very effective, and you may want to investigate getting that installed..
If you go with a ventilation system, I agree with the suggestion to get a true heat recovery system.
In Nelson I would recommend the local smart vent agent. They did our previous house (1950's stucco) and the system they put in was excellent. We also got them to replace an ineffective bathroom fan in our current place and they did an excellent job of that too.
Personally I would ventilate before I double glazed. We observed more benefit from ventilation than DG. The ventilation allowed us to drop the heat pump temp by a couple of degrees for the same comfort level.
It's especially effective if you have a room in the house that gets a lot of sun, as it distributes this heat around the house.
I disagree with the builder's opinion that DG reduces solar gain. While it does reduce incoming light somewhat it traps more of the heat that radiates off objects that light touches. Our current house has DG and can be oven-like in summer if heat is not managed with ventilation (note we have dark coloured floor surfaces).
You are almost always going to get best bang for buck installing more insulation. Double glazing only increases the R value by a fraction, and you will still get a huge amount of heat loss out the glass. For ventilation, open a window, far cheaper. One of the big problems in NZ with mould etc, is due to people not opening their windows. They cause all this moisture to build up, but then seal their houses with the door closed. Natural stack ventilation is very effective, and you may want to investigate getting that installed..
Its true that dual glazed windows wont have anything on any pink batts for the relative R value but this only part of the equation old windows tend to let breezes through the gaps we used to have thermal curtains closed, draft stoppers in between the joins on our wooden windows and we still could feel breezes on a stormy night. Once we changed to dual glazed uVPC the cold breeze you could feel disappeared and we got better sleep at night not hearing the storms anymore.
For us no breeze and condensation on the inside being a thing of the past made it well worth the cost of the window replacement probably the best thing we did.
Basically I would do things in this order:
After that - see if you really need a ventilation unit if you do - buy other than HRV and get a true unit that captures heat from any air exiting partially warming up colder air coming in rather than the positive pressure units that are nothing but glorified fans on timers. After we insulated our house we pulled our old HRV unit that came with the house out.