g'day
Metri will charge an extra 20% on downlights if you request them, but that may be a better option depending on how your chosen electrical contractor is going to charge you to install downies on a finished house.
if you are not replacing the current fittings with downlights you will be left with a small 20mm hole with 2 5mm holes either side, if the electrician is neat
any decent electrical contractor should be able to run the extra lighting wires you need without the need to cut holes..
in your situation you may encounter the following problems:
all downlight positions are dependant on where the roof trusses are located, plaster batton locations ( if you have them ) and physical ceiling space. It is possible that one may cut a downlight hole and have a nice big load bearing roof truss staring at them.. so i would budget for some patch and paint into your upgrades.
depending on how many downlights you want to install, the current wiring may not handle the extra load 40 downlights can place on a lighting system designed for 1 light per room.
so you may have to factor in an extra run of cables to your switchboard, an extra safety switch. etc etc. etc.
its a hard one... i would go direct to the electrician that is doing your house and ask them for a quote on downlight prewire off your existing lighting. that way he can install the wiring you need, and you dont need to cough up the extra 20% metri charges for extras.