in my experience and also my impressions from anecdotal information from cohorts and forums, I'd highly recommend a used or refurbed 7880. Of all the LF Epsons I've owned for either OEM inks or Cone, it just works day in and day out, and my experiences with it seems consistent with others. Jon was even buying and warehousing x880s before Epson discontinued and sold out. Reports about x890s, and x900s is less encouraging.
You'll need an 8 ink printer, so a 7600 would be less than ideal though you could probably find one for all but free. They are getting pretty old though, so condition may be an issue, on the other hand they are easy to work on. So the oldest printer appropriate would be the 7800, a used one should be very affordable, and they work well. My 9800 was ok, given the usual Epson bad days. The x880s though, though very similar, seems to have way less bad days, mine, essentially none year after year. Though very similar, the teflon coating of the heads and whatever small changes made effected ink delivery performance in a significant manner. A 7880 should be reasonably priced used or refurbed.
Things changed worth the x890s, the pressure system and ink path becomes more complex, reflecting the new approach committed to in the x900s, and seems to have more associated complaints. Still, I'm sure many work well and in would be appropriate for the setup.
Personally, I'd stay away from a 7900 because compared to the older models, there are more reported ink delivery problems. On the other hand, if you want to wait a bit, Cone is coming out with a setup for the x900s in 2015 utilizing the additional ink sets that will offer more hue flexibility.
Hope that helps
