Alternative Index

Hi,

I need to add an index (or a post ID, a string identifier of some sort) to every post, I couldn’t find any plugin that do this function, so I guess I will have to do some coding myself.

What I’m looking for is really simple, every post will have an additional field. This field contains the index I mentioned previously, and the index is basically a string of letters and numbers. The string of letters is associated with the category, and the number should increment with each new post.
By default, it should automatically assign an index to every new post, but also allow user to change it as long as it doesn’t collide with an existing index.

I have some thoughts about how I can do this. From the database perspective, I should create a new table that associate each post ID (assigned by WP by default) with an automatic index.
However, I have no idea what should I do from a PHP perspective. I mean I need to somehow show the index associated with a post, preferably somewhere in edit.php. And how can I verify that the user is not manually entering an index that already existed (run a SQL search or something)?
Also, how can I associate a prefix with a particular post category?

Angela