Brown water means rust! The block is cast iron and no antifreeze means it will rust, and the alloy head will corrode, not good!

Remove header tank cap. There is a bolt in the block on the exhaust side near the rear of the engine (17 or 19mm i forget now). You remove this to drain the block of coolant. Then remove the bottom hose of the rad to drain that. Remove the top hose and remove the thermostat (buy a new one when you buy your ANTIFREEZE!) and then use a hose pipe in the end of the housing to flush the old crap out of the drain hole (until it comes out clear). Then replace the bolt in the block. Then put the hose in the bottom rad outlet and flush it until clear water comes out of the top hose, then put it in the top outlet until it comes out clear in the bottom outlet. Replace all of your hoses and thermostat. Then remove the bleed cap hose on the thermostat housing, and slowly fill the system up with a 50/50 mix of good quality antifreeze and water. When coolant starts coming out of the bleed point replace the bung hose and continue to fill up the header tank to the max level. Then turn on the engine and let it idle with the cap removed until the fans have come on and gone off. Leave to cool (could take a while!). Then replace the header tank cap and again turn on engine and leave to idle until fans have come on / gone off. Check for any leaks and if happy thats job done