Determining the features of the DSP
This doesn't sound like you need a DSP at all.

You'll need something that does USB3 towards your computer. Typical solution to that is the fx3 chip from cypress. I'm not aware of DSPs that integrate a USB 3 phy or even just the logic to speak to one - not a typical interconnect in their world.

Then you need something that orchestrates the ADC, keeps timing correct, and hands over the data to the usb3 chip, but doesn't do much of actual signal processing.

The whole "glueing one high speed digital thing to another" aspect screams for an FPGA. So, what I'd recommend: pick an FX3 development board and design an FPGA/ADC add-on board to that.

May I also interest you in other interconnects? Maybe gigabit Ethernet is sufficient for your use case; in that case, you could basically use the Ettus N200 as reference design (schematics are public). Or buy one - it comes with a dual-100MS/s ADC, and with the BasicRX daughterboard, you can directly interface with that ADC in baseband. 

Full disclosure: I'm affiliated with Ettus, but to be honest, I'd like to see you succeed in building your DAQ device :)