In this paper, we investigate opportunistic buffer-equipped, multi-antenna relay networks with a decode-and-forward relaying strategy. The channel that we are considering includes three key wireless channel effects: path loss, shadowing, and fast fading. We first analyze the direct communication outage probability, and based on that, analyze the opportunistic relay network's outage probability. For an opportunistic relay selection protocol, we consider max-max relay selection (MMRS), where two relays are involved in the relaying: one for the source data reception and the other for transmission to the destination. As a relay selection method, we suggest dual-timer relay selection (DTRS), which reduces channel estimation overhead and solves the full-buffer problem of a reception relay and the empty-buffer problem of a transmission relay in the MMRS protocol. Along with the DTRS protocol, we further investigate the optimal relay selection period for maximizing the throughput while avoiding selection overhead.