Malaga used to be a good place to hire a car in the off season. You could get a Kangoo or Berlingo for around EUR10 long term then take the back seats out and sleep in the back. Not sure how the prices are now but it's easy to check online. Keep in mind El Chorro can be accessed by train if need be.
El Chorro is awesome in the 6's and 7's. Get the Rockfax guide and you can't go wrong with any of the 2-3 star routes......there was enough to keep me going for weeks. The weather is obviously more reliable the further south you go. And i've rarely had a wet day in Chorro.
I've done Siurana and Margalef in March/April and it was nice and sunny but have heard the odd story of lots of rain and/or snow in Dec/Jan. They are quite high elevation so will be quite cold in winter. Costa Blanca is also a good winter sun option with plenty of good routes in your range.
Good places to stay are La Finca and Olive House in El Chorro (walk to crags), Orange House in Costa Blanca (need a car) and Camping Siurana (Toni Arbones place). There are other refugios all over the place near crags. I wouldn't bother pre-booking except maybe for the xmas/new year stretch.
I have always flown easy jet from London to Malaga/Alicante/Barca so not sure how you would do it direct from Oz.
PM me for route recommendations or more info if you need.