I have been looking into customizing the WP dashboard / backend area to include some business management features. A long time ago I custom coded a time tracking system & invoicing system into my site ( it was done in cfm not php ). I hadn't been using it in a long time, so the feature was depreciated when I relaunched on a new server. I am now looking to add this back in.

I would like to (in order of importance):
- Manage a list of clients & track time, project costs
- Create invoices
- Connect that to a checkout system. I could link the client to a page where they can see and pay their current balance.

Because I would like to offer a checkout system, I was thinking I could make use of one of the ecommerce plugins then create carts they could login and see? Just checking to see if anyone has done similar things or come across any solutions that might work here.