If I had a choice between a 3880 and a P800 for B&W using third party inks, I'd choose the 3880, especially if it really is unused. Partly because of the chip lockout issue, which seems to be a non-issue with the 3880, and partly because of the lack of a proper rear feed on the P800, and being forced to use its cantankerous front-feed path (even though it is still possible to rear feed using the roll feed path).
The factors that would have me consider the P800 would be the greater ease in using roll paper (you have to cut it into sheets on the 3880 and need to use a RIP for long prints), and perhaps the MK / PK switch. That switch is the Achilles' Heel of the 3880, and so far there are no reports of failure that I've seen for the same failure on the P800, although it may be still a bit too early to be sure. If the 3880 really is unused, then you've probably got several years before this starts to become a risk.
Of all the printers that you listed, I'd choose the P400, because these carts-on-head printers ars easier and cheaper to maintain, hibernate, and switch inksets, provided that you can settle for 13" rather than 17". There have been recent reports of Epson trying to force P400 firmware upgrades on people, so you still need to be alert and wary, but that is the way of the future I fear.