An Alternative Approach to Automated Testing
It would take an awful long time (and cost) before you've caught up with the code. Mention this to stakeholders/managers and you could be derailed before you've even started.
I've recently heard of an alternative approach that sounds quite interesting and it wouldn't take too much effort to get automation up and running.
Let me explain - Instead of trying to automate each module, you start with automating defect fixes. The idea is that after a defect has been fixed a verified as working correctly, a small automated script is written to cover the functionality. Over a period of time, the suite slowly starts to build up into a regression pack. At fist the coverage is low just as with it would be following a traditional approach. However, as you're testing bug fixes, these tend to be around the most problematic areas of the code. Also by automating bug fixes, you can easily find problems when software code has regressed.
As time goes by, you could start to look at introducing a more module like traditional approach. But as a starting point, I think this could be the answer to introducing automation with quick results.