It runs on HD cards also so see if someone is selling an HD 1 or HD 2 with Ilok auth, HD native is the smarter long term but with a License will run somewhere at 2.5k and up. I would also think about thunderbolt interface but like SSL says, "It aint cheap".
I must say the D to A is very nice on the omni. I mix and I am amazed at the difference in the sound compared to a 192. Its a good box with a confusing I/O. Its also has a built in, up to 7.1 monitor controller. Very useful box if you don't need a lot of inputs. It does function as a standalone box with a mixer input for the 4 analog and 4 digital inputs. You could run a spdif or AES output into the inputs and output to the main output as a monitor controller without the use of an Avid card. It's confusing encrypted front panel settings for setting up. It's easier if you have Protools to get to the Hardware setup page but can be setup from the omni if it has to be.