Water pump, tensioner pulley and timming belt every time always together NEVER do just one. The reason is once you release the tension on the belt and tensioner pulley (held in place by the water pump) is drastically reduces the life of the belt and bearings in the tensioner. Might sound a bit extreme but a 100 bucks worth of parts every other year or 30,000 miles plus the reccommended oil changes and services will make that engine go 300-500 thousand miles. I have 2 e30 cars ones with 286,000 one with 220,000 and both run like tops and are uber reliable. As far as filling the coolant, turn the heater to the "on" position (full hot on the selector knob), this ensures you fill the heater core. Top up the coolant (might not be a bad idea to throw in a thermostat and clean the housing so there's no obstruction in the bleed passage while your at it) and start the vehicle. open the bleed screw enough to let the air out but DO NOT remove it from the water neck (bad things happen). You may have to open and close it several times for about 15 minutes to get all of the air out of the system (engine running up to operating temp). Top up the coolant and keep an eye on the gauge and coolant indicator light for a while (several days of driving). It's not uncommon to do all of this and several days later an air bubble will escape and it'll need a little more coolant added. I replace mine a little more often than recommended (60,000 mile interval 3 year etc.) But I'm also NEVER stuck on the side of the road with a broke belt, bent valves, and a lot of new fun words I've invented......knock on wood.