Any radio that has a single camera input will work, as your Greyhawk has a switcher in it. It monitors the blinkers & reverse to display the correct camera.
I would highly recommend the DNX995S as it does everything the Soundstream does and is Kenwood's flagship radio. From last year. So it is severely discounted, about half off. You can get it for $650 from Sounds of Tri-State. One word about buying car stereos, though- you can find them cheaper, but if it isn't an authorized dealer, they won't warranty it. You also run the risk of getting a return/used unit or even a counterfeit one.
It isn't a complicated install, but there is some weird wiring there. Most car radio shops will install gear you brought yourself if you don't want to tackle it. I had my radio, a dashcam, and front and rear speakers installed on my first trip. That was around four hours. I felt it was missing the low end so I had a sub installed under the passenger seat and it took around three hours. I wasn't going for loud- I was going for sound quality and my radio sounds extremely good.
Here is a video that shows my older radio. I swapped to a 995S as the Bluetooth module in the 875S was being weird. You'll also see the rings the installer had to build to make the rear speakers fit. I am not sure if you'll have to deal with that in your Greyhawk, but something to be aware of. It also shows some of the little things we've done to our 22J. Since I've shot this, I added a bedroom TV, dropped the bug screens and went to flea collars, and as mentioned, swapped the radio.