Is it possible to find a spider and cockroach in an apartment in Sydney and Melbourne?

If the apartment is kept clean there will not be any real problem with insects.  Don't let stories of Funnel web or red back spiders worry you as the fact is the poisonous creatures are very rarely the cause of death here.  I doubt anyone has died from a spider in Australia for 30 or 40 years or more.  Any house in any country can have a spider or other bug in them and Australia is probably no worse than any other country.  Most visitors to Australia never see any spiders or snakes and definitely not in the cities.  I would say there no more bugs in apartments in Australia than in homes in Brazil.

yes especially a cheap one

Very much so.  But I wouldn't worry too much about the native cockroaches. As for the spiders there's several deadly species around so take care!   Also ALL the species of snakes you'll find in Melbourne and Sydney are deadly!

It's certainly possible, yes.

You live in Brazil? There will be at  least one  live  spider within  2  metres  of  wherever  you  are  sitting.

i dont see why not. theyre everywhere