A reliable office network starts with structured cabling. Cat6 runs terminated on a patch panel inside a proper rack are easier to fault-find and far more durable than ad-hoc cabling.
Place access points based on a site survey rather than convenience. Wall and floor materials change coverage dramatically.
Keep the rack ventilated, powered through a UPS, and label every port and cable.
Plan for redundancy where downtime is costly: a secondary internet link and spare switch can pay for themselves in one outage.