As Mark says, if you find the tool really useful, buy it. Personally, I use ReSharper to run my tests, but this is a personal choice.
I also use NCrunch , which is free (and wonderful). I believe that you can also use this for debugging.
However, you can debug using the NUnit GUI - just attach the debugger to the process, set breakpoints and run the tests.
However, I usually try not to require the use of a debugger to fix the tests. When a statement is not executed in a well-designed test, this usually provides enough information to fix the implementation, after some thought. If this is not the case, this involves a complex script that sometimes suggests adding more tests to process each part of the script. But yes, sometimes you need a debugger ...