Before you buy a used Powertap realize that you can get a very nice Powertap rear wheel custom built for ~$800-900. Then you can get a matching front wheel built up when the funds permit. Used Powertaps are somewhat of a gamble because if they break it's $350-400 to have Saris repair them (and new they go for $700-800). If it were my money buying now I'd get this:
HOOPs Wheels - HOOPs 2431 Elite Kinlin 31t Wheelset