What ADC requirements for 20 kHz to 10 MHz signal?
If you want to read a 10 MHz signal, then you'll need a sampling frequency somewhat above 20 Ms/s. A little bit above if you have a very good anti-alias filter, a lot above if you'd like an easier to design filter. I'd recommend nothing less than 30 Ms/s, and even then the filter will be quite challenging.
Very little degrades at low signal input frequencies. Most ADC parameters improve as the signal input frequency falls. The most demonstrable of these is the effect of phase jitter on the sampling clock. This converts a rapidly changing input voltage directly to noise.
At very low frequencies, we run into the rising 1/f noise of all semiconductor systems, though at 20 kHz an ADC should still be very quantisation noise limited rather than 1/f.
The best way to find an ADC for a specific purpose is to go to the usual manufacturers, Analog Devices, Texas, Maxim, and use their online parametric search tools. Or to find something from across all the manufacturers and have price and availabilty readily to hand, use the Mouser or Digikey parametric search tools.
There's no way any of us could make a useful recommendation, even if it weren't against the site rules (as recommendations for specific products would age very quickly as new parts come to market). For your particular application you'd also need to factor in considerations like the data interface (serial, parallel, LVDS, CMOS, JESD, it matters at 30 Ms/s 16 bits), power consumption, cost, package, single ended or diff inputs, on board reference, and of course noise, spurious, drift, accuracy specifications.
Happy searching.
You've mentioned the ADS4145 ADC at 125 Ms/s. That's a fine device. It will read 10 MHz and 20 kHz quite happily. There are two things you can do if you only want to read to 10 MHz. One is to reduce the clock rate to the ADC, meaning there's less output data to handle. The other is to decimate the output data digitally, meaning you'll get a small improvement in noise through oversampling, and a much easier to design anti alias filter. If you're happy with the tiny device, the DDR LVDS output, and the price, there's no reason not to use it.