A 58-year-old man with longstanding type 2 diabetes presents to the emergency department with polyuria, polydipsia, nausea and abdominal pain. He appears dehydrated and tachypnoeic with Kussmaul respirations. Capillary glucose is 27 mmol/L. VBG shows pH 7.12, bicarbonate 10 mmol/L and ketones 6 mmol/L. According to standard DKA management protocols, what is the most appropriate initial fluid therapy?