New User management System - How?

Hi,

(it is a newbie question)

I have to build a new user management system (UMS) to my company that
will be used for all systems (ERP/CRM, Web Site, etc).

My first approach would be build it from scratch, but after small
research I found out that I could use some resources from .Net
Framework and others Microsoft technologies.

1. The first thing was Extended Rights Reference
(ExtendedRightAccessRule) from AD. It seems that I could create
permissions (like "Can modify customer info") and associate it to user
or a group. But it seems a little bit tricky to create/configure these
permissions and, because some users of the system aren't on the same
AD tree, it doesn't look the best approach.

2. Later I found System.Security (AccessControl, Principal, etc). It
looks like I can fit my UMS together with those Interfaces and Base
Classes. For example, I could have a list of users and permissions
from my database and create my own UMS implementing these interfaces
and classes, so it would fit the .Net archicteture.

Is AD a good repository for user management for other system than
Windows/Exchange?

Is it possible to use those base classes from .Net framework? If yes,
does it worth?

.