Look what I'm putting my 4T60'* through. They hold up well, and can be built for the power. Just remember when modding, any car will need a trans sooner.
The good news is that we've not seen more 96 SSEi'* with failed transmissions, even though they got the older 4T60. In fact, if anything, FEWER Series 2 L67'* mated to 4T60'* fail than Series 1 L67'* mated to the same trans.
The Series 1 develops it'* torque at a lower RPM which tends to be more abusive to the trans than the S2 in that regard. I wouldn't hesitate to buy a 96SSEi as long as the car checks out.