Oscilloscope input circuit
Input capacitance is significant for capturing high frequency content signals. Given that you are using discrete parts and not considering RF parasitics, this system will be restricted to the 100's of kHz for 1% accuracy.



The goal is to maintain constant attenuation and input impedance over the entire frequency range. Assume Cp is 100pF, Rp is 941kΩ, Cs is 1000pF and Rs is 49kΩ (ignore adjustment cap for now) The capacitors' impedance will vary as follows:


ZCp : 1.6GΩ @ 1Hz; 1.6MΩ @ 1kHz; 16kΩ @ 100kHz
ZCadj. : 16GΩ @ 1Hz; 16MΩ @ 1kHz; 159kΩ @ 100kHz
ZCs : 159GΩ @ 1Hz; 159kΩ @ 1kHz; 1.6kΩ @ 100kHz


This results in the following range of impedances:


Zprobe : 940kΩ @ 1Hz; 591kΩ @ 1kHz; 15kΩ @ 100kHz
Z'scope : 49kΩ @ 1Hz; 37kΩ @ 1kHz; 1.5kΩ @ 100kHz
Zinput : 989kΩ @ 1Hz; 628kΩ @ 1kHz; 16.5kΩ @ 100kHz
approximate attenuation: 20X @ 1Hz; 17X @ 1kHz; 11X @ 100kHz


As you can see, adjustment capacitors are required to tune the input. The grander issue is that this calls for wideband matching networks. Another option, and what is done in addition to matching networks in professional equipment, is characterizing the input impedance and compensating for irregularities in software. Standardizing at 50Ω impedances allows the construction of modular probing hardware while maintaining wideband matching and constant attenuation.

Your requirements for switching impedances and an optional 50Ω input will need some more complication. A fantastic project, and I wish I could partake!