You've three choices about where you cross the WA border, two are sealed roads and good for inexperienced travellers. The third is to use one of the remote roads this should only be done by experienced drivers, as you'll need to be self reliant for a number of days, I'll ignore this option.
Option 1.
Going via the north is the longer journey and will add significantly to time and cost. It well worth the experience and the option as to how you journey down to Melbourne are infinte.
Option 2.
This is the one I recommend it the shortest straight line crossing.
Form there its just a question of what interests you
Perth -> Kalgoorlie -> Eucla ->Port Augusta -> Mildura -> Melbourne ... fastest route
The Perth Albany Esperance Port Augusta Adelaide Mt Gambia Coast road will give you the greatest scenery and the most beaches.
What I think will provide the greatest experiences is Perth - Albany - Lake Grace - Hyden - then follow the unsealed roads across to Kalgoorlie, this route is well sign posted as a heritage trail and follows the route of the early prospectors, an opportunity to camp for one or two nights in the "Bush".
Crossing the Nullabor follow the sealed road, Norseman -> Eucla -> Ceduna. but there are many varied opportunities along the route if you have time take the detours to the coast and also consider a day trip upto one of the station on the rail line (Zanthus is my favourite). Once at Ceduna spend a few days exploring the coast before moving on.
Once on the move make the detour to Coober Pedy from there head to Port Augusta, consider spending some time exploring the Flinders ranges. From there Adelaide, Mt Gambia and the Coast road is the best. When you reach Geelong take the ferry across the bay from Queenscliffe to Sorrento.
Voila your in the south eastern suburbs of Melbourne