Depending on what hardware (aka what company your account will be with) then the wifi should not be an issue. Bog standard Optus equipment performing exceedingly well for us - SAGEcom 20cm high rectangular block.
In our case - every internal wall is double brick down stairs. In one case there are 3 x double brick walls between the modem & recipient - no speed issue at all including high def Zoom work meetings while two other locations streaming video that are each 2 x double brick walls away.
Unless the floor/ceiling is solid concrete with much rebar - then upstairs unlikely to be any issue.
BTW: this is on a 50 plan.
As for wifi issues - when I check 'available networks' on my US $149 'Black Friday' 2016 laptop - there are always at least 9 other wifi networks showing from adjacent houses & across the (4 lane road). Signal strength for all but 2 of the normal 9 is 80 to 100%.
So it would seem that, at least in our case, the wifi issues may be more to do with what hardware is supplied rather than the house construction.
