With no level bracket increases (+/- 100 levels): 9 targets (Pretty dead considering a cooldown of 10 days per guild)
With the suggested +/-25 levels per 1000 level addition (+/- 175 levels): 18 targets (Much more acceptable)
On the offline time, I assume you plan to follow the current behaviour. Right now, a player can become inactive during the conflict. So if you start a GvG, and someone in the defending guild has been offline for 6-7 days, at some point during the 24 hour period (assuming the GvG lasts the full period) - they will become afk and unattackable during the GvG.