You can already create an invoice, quote, or credit note from the client list (there are little icons on each line to do that).
You can already email or print an invoice, quote, or credit note from their respective lists.
What would you like to see on the 'statement'? The nBill module does show a summary of orders and invoices, including amount outstanding and a link to pay all, so that might help.
Recurring invoices are managed and controlled from the order list. Just saying 'improve' doesn't really address what you would like to see done differently...?
Receipts can be printed from the back end, and the ability to email them and view them from the front end is on the 'to do' list. Some of the framework is in place for purchase orders, but I ran out of time to include them in nBill 2.0, so they are on the back burner for now.
If you can elaborate on what changes you would like to see, I will of course consider them for future development.