The foundation is the most important component. It must be strong enough to support the building without breaking or sinking.
Providing a compacted sand base for your home's foundation is not a bad idea. It offers a level, firm base with good drainage.
A concrete foundation, with metal reinforcement, is perhaps the strongest base to give your home. It is preferable for most forms of building.
Some forms of building, like earthbags and rapidobe, do not require a concrete foundation. For these, a rubble foundation is fine and will save you considerable money.