If you don't have automated tests, and start writing them as early as you can, you're not going to have a good time. I hate working at shops or on jobs where there is zero unit testing. You have to have tests to safely refactor things. If you miss something, you add a test. If you get a false failure; figure out what was wrong with the test.
Retesting should be as simple as running the test scripts. If it's not automated, it's not tested correctly.
I'd rather have automated integration or system test and no unit test than the other way around. I find unit tests uncover the fewest problems of the three.
If you don't have automated tests, and start writing them as early as you can, you're not going to have a good time. I hate working at shops or on jobs where there is zero unit testing. You have to have tests to safely refactor things. If you miss something, you add a test. If you get a false failure; figure out what was wrong with the test.
Retesting should be as simple as running the test scripts. If it's not automated, it's not tested correctly.