10pf at 100khz has an Xc of 159k, so a 100k input resistor would be about the highest resistance you could go. the -3db corner frequency of 100k and 10pf would be 159k. so you might get away with a 100k input resistor with extreme caution in your board layout, but i wouldn't think you would want to go any higher than that. there are few things you would need such a high input impedance for, one of them would be a piezo sensor, another would be a crystal radio preamp. since you're looking for 100khz response, my guess is a piezo element.