First DAC: Metrum-MHDT-Rega-or?

I have read up on a lot of the posts here. Not sure if that led to more or fewer questions. I have a decent system, modded AR-XB table, Lehmann Phone pre, Exposure integrated and CDP, Harbeth speakers. I want to add my first DAC to the system. Initially it will take iTunes data from a MAC and run into the Exposure amp. I may also run digital out of the CDP into the DAC to see if that improves overall quality. I would like to stay at around $1000.THat is not a lot to some of you but that is kind of my limit at this point. From some of the reading I have done I am leaning toward a Metrum Amethyst, MHDT Orchid or Stockholm, or a Rega DAC-R. I am also open to used units as well. I do not want Pre-amp functionality, and a headphone amp is not critical but nice to have. I am interested in your commentary on the above units and brands. In addition the MAC is more than 15' away from the amp, so I am going to have to piece together USB, Interconnects and maybe SPDIF cables. Is there one cable I should avoid using in long lengths relative to the others?

I admit I am a total newbie in this. I will learn more about software...etc, but now just trying to make an educated decision as s first step.

Thanks