Very good points made by the above poster.
It is always riskier buying a used projector if you cannot verify it first, because you have no way of swapping it out with a warranty if you find a defect in the image.
Agree, that shadow detail is partly a function of gamma, it is also partly a function of the processing of the specific projector, as well as any IRIS difficulties, etc (of course the JVC do not have IRIS's).
JVC is certainly a good choice, but I'm not sure if you can even find any HD250's or any new JVC right now under 3.5k to 4.5k. JVC has a stock issue on almost all of their lower end models and are hard to get last I heard.
The motion is really only an issue if gaming, IMHO they are for the most part all close enough these days, it might matter a little in action movies or if you watch a lot of Sports, depends how picky you are.
The Epsons POP just a bit more than the Panny. Another alternative is the Sanyo z4000, it probably has more POP than either of these other LCD's for bright scenes (the Sanyo is sharper and has higher ANSI contrast), but it is not bright enough for most. Other than that, check out some DLP's maybe.
As far as the technology variance goes:
It depends on the specific projector just as much as the tech, some LCD's have more POP than others. In general, DLP POPS a little more because of sharpness. I don't think DLP has better colors than LCD, maybe a tiny bit better "dark colors" in shadow detail, LCOS has better colors than both though just by a tad. All 3 technologies are VERY close when it comes to color, it's more about the projector itself as far as color goes.
Image processing and noise reduction is IMHO one of a few underrated aspects that varies between projectors which is often ignored by the reviewers. Although that said, for the most part our eyes adjust to the varying image noise and ignore it, but if you do a side-by-side, some projectors are clearly better at this than others.