I would use I2S and interface with any ADC/DAC board that is fit for the purpose. You can find many third-party ADC/ DAC boards with I2C interfacing. That's more flexible. You may also add SPDIF and/or optical I/Os.
I don't know the TI DSPs very well, and the ADSP line is rather good, so I'd tend to favor those, but I'm biased. One thing to consider before choosing is the development tools. How expensive are they? Etc. As far as I remember, the ADSP dev tools were expensive. That may have changed.
As for the Rigol oscilloscope, it's more than adequate to debug your board and see what's going on. Don't expect to use it to precisely measure distortion figures, though.
I don't have ADC/DAC boards, but I have the chips.
Will this arrangement for ADSP-BF531 work, using Raspberry Pi as the optional host controller and one of the I2S targets, and a codec on the other I2S:
* Form factor: Raspberry Pi HAT
* 64MB SDRAM (MT48LC32M16)
* Signal switches for selecting SPI master/slave boot modes (so it can work with or without the Pi)
* CS4272 or WM8978 on one of two I2S ports, controlled using SPI
* Raspberry Pi I2S on the other I2S port
* Boot ROM device: W25Q128FVSIG (16MB SPI)
* Should I just shut the PPI in, or what should I bring it out to?
If I get the Pi involved it can be used to inject code (using the slave SPI port on the DSP) and test data. Actually on the Pi there is shairport-sync software that implements an AirPlay target, allowing me to stream music into it from my iPhone. With a DSP in the middle it would be interesting to meddle with, for example, those Taylor Swift songs.
Also what is the TMS320VC5509 equivalent?