It may be possible to read at 100MHz for long enough to take a useful sample, but getting a 100MHz ADC working well will be a challenge I think, you won't be able to play with is on a breadboard at that speed.
I played with a similar idea a while back, I eventually decided to use hardware for the sampling into external RAM and a fast uC for triggering etc.
That said some processors have a camera interface (I think that's what it's called), most notably the PICs. AFAIK this is designed to read a parallel port directly and DMA the results into memory. That may be a good option.
Rob