I have just bought a Yamaha YZF R6 2000 19K miles for a low price because of engine rebuild!?

The distilled water as coolant is a very very bad idea. Distilled water is hungry water and will actually begin to ruin your cooling system. 50 /50 using approved coolant and water from the spigot will be fine. The absolute best accessory you can buy to make the bike last longer is a shop manual. Follow the maintenance procedures, don't make your own up and don't follow internet advice. That will make your bike last longer.