You do not have to pay every month, you pay once then get the rank forever. To rank up from iron, on the donations page click the "rank-up" section, then find iron to diamond.
With diamond rank, you can vote for basic, normal or OP chests. You can also vote for time of day.
You do not get all the kits, you still have to buy them as any other player would.
You can have grey, dark blue, yellow or cyan glass.
Diamond is a lower rank than obsidian, as it only costs £70 whereas obsidian costs £150.
It will tell you the differences between each rank on the donations page I believe.