pink coolant is organic, and I dont know of many radiator places that recommend it. Green is glycol based. Idea behind pressurising the cooling system is, to increase the boiling temp of the water. from memory it boils at 113 under 13 psi of pressure. on any efi car it is a bad practice to run any other temp thermostat than recommended as the ECU relys on coolant temp to adjust fuel mixtures and so forth, too cool a thermostat and the ECU things the car is cold so richens the mixtures up and you use more fuel
Most radiator shops can do a clean out on the radiator for around $100 mark, it pays to shop around as the prices can vary. That is assuming you remove the radiator.
Premix coolants are not a good idea unless you rebuild the motor. Most cooling system will retain water in them, some Subarus hold nearly 1/2 of the water still after you drain the radiator.
Best idea is to flush the cooling system till clean water comes out, then drain enough water to refill with 5l of concentrate(most E series hold 15l in cooling system, and most glycol based coolant you mix 1/3 coolant 2/3 water) then top up with water. And yes distilled water is better, but nearly impossible to flush entire system out as described before.