Creating a complex mocking infrastructure will probably just hide the problems in your code. I would recommend that you start with integration tests, with a test database, around the areas of the code base that you plan to change. Once you have enough tests to ensure that you won't break anything if you make a change, you can start to refactor the code to make it more testable.
Se also Michael Feathers excellent book Working effectively with legacy code, its a must read for anyone thinking of introducing TDD into a legacy code base.
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