I have been using PlayerRankins@Ankawi with Kits for several years and it works very well,
You would have already created your groups and would have designated what group = what rank (= how many hours on server) so all you do is create kits with permissions and assign those permissions to which group you want to have each kit,
When the player reaches the required time they would achieve the rank and when opening the Kits gui it would show the kit as redeemable.
If that is not clear enough please reply with your concerns