If you find something you like that needs a low source impedance, you can always put a high impedance buffer as a frontend. If your buffer stage is either unity gain or low gain, it doesn't need nearly the gain bandwidth product as your main amp, so it should be easier to find something that's low noise and has very little current requirements from your probe. I definitely think cascading amps is going to be your best bet if you're needing several MHz of bandwidth and very low noise - most of the amps with several hundred MHz GBP required to get good gain and frequency response in one stage generally have higher noise figures near DC (audio amps are often spec'd at 1kHz or 10kHz whereas faster amps are spec'd at 1MHz and the noise figure generally goes up exponentially towards DC), so while they'd be fine as a high gain stage for a reasonably fast AC signal, you're going to lose the low level fidelity near DC. Using amps designed more for low frequency use will give you much better noise performance (and when looking at very small signals, this will be important), but will generally have much lower gain bandwidth product, giving you much lower maximum gain for a given bandwidth requirement.
It's also probably worth including a low pass filter for your intended maximum bandwidth in your frontend - reducing the overall bandwidth of the system inherently reduces the noise floor, so limiting it to the frequency span you want to look at will improve low noise performance. Looking at the very small signals also means you're much more susceptible to grounding issues, supply noise, and EMF.... so be careful the way you layout the design. Local regulation, ample decoupling, an analog ground plane with a wide single path to ground, and maybe even shielding could be beneficial to the overall performance of the design.