Creating a Rank System - How to rank players up through PEX?

Basically, I am creating a rank system. This simple rank system is a reward for my hunger games users. For every kill, they will get economy money. Eventually, they will have enough money to rank up. Here is where the problem comes in.

I want a plugin that basically executes a command when you pay economy money. I need it so that they can simply use /shop and see how much the rank costs. Then, when they use /buy (Item ID), it will buy that rank.

Here is the tricky part. How in the world can I make it so that they have just purchased the next rank in line in pex? I need it so that they automatically go to the next rank, and a new one displays in the shop for the next rank.

Thank you,
Mike

Edit: It needs to be a dynamic skill-tree like system. When you buy a rank, the next, better rank appears on the list automatically, and the previous one you just bought is removed.

 

Rankup the plugin does the ranking that costs money then you would just need to find a plugin that converts kills to money.