I didn't bother with the engine block drains. I just drained the radiator *****, and got about 8 litres out, which for a 9.1 litre system, I though was acceptable.
As for the coolant, the OEM coolant is very expensive, but there is no equivalent available on the market. That said, I don't think there's that much harm in a generic coolant like a G30, which is likely going to be fine.
However, it is worth remembering that Japanese manufacturers all use the same formula these days, and it is totally different from anything designed for European or American cars. You can often find "Mazda FL22" coolant available for pennies, it's the same formulation that toyota use from the same OEM (CCI), but dyed green.
The coolant provides 3 functions: cooling (provided by the water in the coolant), protection against freezing (provided by ethylene glycol), protection against corrosion and water pump erosion (provided by the inhibitors). Water and ethylene glycol are the same in all coolants.
Japanese OEMs use phosphate inhibitor because of enhanced protection of the water pump from erosion. The phosphate is boosted with organic acids. For environmental reasons, European OEMs use phosphate free, pure organic acid inhibitors.