[timwiseman]

Even then I would approach it with confidence. You can learn things very quickly by doing, especially when highly motivated and especially if you can budget some extra time (perhaps unbilled?) to account for the learning curve.

I would only turn down work for fear that you are not capable if you know it is truly beyond your ability for the forseeable future or if the deadline is too constrained to permit yourself to recover from mistakes or delays from learning.