I wonder what you hope to achieve via the independent - are you keen for great results, the broader extra curricular or both?
If you are wanting academic excellence, then the non-selectives might not offer much more in the way if academics than a state school - what really would you be paying for? You clearly see the non selectives as a second best option - so are they actually worth paying for at all?
I would agree that you can certainly do Infants in a State school to save money. Lots of people do 'State 'til 8' and then move their kids as pre-prep in Indies corresponds with INfants and Prep begins in Yr3 as Junior starts.
In many Indies, it is in Prep from Yr3 that the subject specialists really get used, so there is more difference to State than perhaps in pre-prep which will be similar but with smaller classes.
Re the Selective indies - these are what you should really aim for in my view. You have to work backwards to how you can get there within your budget. It should be possible to get there from State if the child is able and had some tutoring. Tutoring, which is targeted at the individual child and towards a specific school entrance exam can be very effective and a focused, cost effective way of preparing, compared to Prep where even with exam prep, a child is still in a class of many and any exam prep might be focused on several schools not just one. The idea that Prep schools put kids miles ahead, in my view, for able kids, is just not really the case......if you look at the very selective day schools, you will see good proportions always come from state schools. Yes, they may have had tutoring and parental input and worked hard, but as a parent you can make all this happen.
If money is a bit tight, I would certainly start in state infant, re-assess at 7 and consider Indie and possibly at State, save all that time for the better Secondary option.
Personally I think Prep is a lovely luxury, but for most people for whom school fees are a real stretch, unless you're in a Grammar area which means you hope not to pay secondary fees, Prep is a lovely luxury which can be lived without and can pretty easily be lived without.....because tutors and parents can plug the gaps and do it really well. Doing this at Secondary just isn't so possible, so it's worth putting the money into that if there is only money for a limited period.