Question for HN - Client/Designer Workflow Webapp

As an owner of a small business design firm I'm always looking for web apps that can help streamline my workflow. At the moment my workflow is done through gmail, with a pretty intense set of labels and filters. I send estimates via email, and wait for replies. I send design 'updates' or ' new versions' via email, and wait for replies. I send invoices via email etc etc etc.

I know there are a few solutions, but I'm having trouble finding one that is SIMPLE enough or deciding if I should take a week to learn the PHP to write my own.

I want simplicity for my clients. I upload an estimate, a new design iteration, an invoice. The client gets an email to log in and view new activity. Client then 'signs-off' to start work, continue work, finish work. Webapp keeps track of history for multiple clients. Webapp has interface for me to easily add new clients and projects.

My question is in 3-parts: 1. Does anything come to mind that has this sort of simplicity and utility? 2. What am I really getting myself into if I build my own? I am an avid entrepreneur, but I've never really got into the bulk of the programmer-centric HN content. 3. Suppose I wanted to try and sell what I create somewhere down the line. What should I know BEFORE I start coding, with this in mind?

I appreciate any time you can take to help me move in the right direction. Perhaps a little project like this may give me skill sets needed for future YComb involvement.

Darren Mills Digital Shift Design

(with such a small text box, I hope there are no HUGE editing errors, cause I'm clicking submit now!)