At this point I wouldn't recommend spending the $ on a power meter of any sort. HR+Cadence will give you enough data for now. There's an Ant+ adapter for the iPhone (about $50). That plus a Garmin cadence sensor and the HR strap you've already got will get you there for under $100. You might want to find a mount for the iPhone, too many options to choose from
When spring rolls around and you get out on the road more an Edge may start looking tempting but certainly wait and see.