I think it is a great idea, it gives the ranks some extra and in my opinion the top ranks doesnt give as much to it, as it could.
As r4ans0m mention, it could be limited to only a couple of times. You could also just make it a one time offer when you buy the rank, giving a set of enchanted gear and tools.
I see no problem with it making a bit of a unbalance between the one who bought the rank, and the regular player on the server. However you are using real money or have earned it the hard way and should therefore also give a couple of awesome benefits.
A one time offer or a monthly kit with a amount of something, doesnt have to be much. Would definitely increase my interest in gaining a rank