Composite video and DAC pin protection
Your question seems to boil down to whether or not you require ESD protection, which is independent of what your main driver is.  Whether you use the integrated buffer in your IC or an external OP-amp you should make the same consideration.  Since you've stated that this is just for testing and presumably in a closed system you can probably get away without a protection circuit.  

However, it's easy enough to implement with an ESD diode 2-channel TVS array.  This has the added bonus of protecting you from reflections if you end up with a poorly matched or terminated line.

As for just wiring the output of the internally buffered DAC to video cable.  There shouldn't be any issue.  For best results you should ensure that you terminate your load properly.  This is especially crucial when dealing with analog signaling as reflections in the system can lead to all sorts of problems.