When I first started planning my theater space I had wanted a Panasonic PT-AE4000U. That was several years ago and there are obviously newer, better models available now. But occasionally I do see used 4000u's for significantly less than the new Panasonic models. But how does it compare picture wise? If i can get a 4000u for say $1000, would you say its still a decent projector for the price, or am I better off just spending the money, even if it is more than double to get something like the 8000u? I'll be upgrading from an old 720p projector I got in 2006.
Just about any 1080p pj upgrade from your 720p would be worth it. Buying used can be ok if you get a good unit, but if it breaks, the repairs are pricey and normally not worth the cost compared to applying the money to a new unit. So your taking a chance with the purchase, vs just getting a new unit with warranty. But is it worth the chance to save the difference. You have to decide.
As to what you can buy in the $1k to $2k range, strengths/weakness/price http://www.avsforum.com/t/1391476/coderguys-top-projector-picks
Regarding the 8000u, I think there are better options for the money, unless you just have to have the lens memory feature, it offers.
BTW, there is a good forum member selling a used 4000 in the classifieds, if you want to discuss it. http://www.avsforum.com/t/1472904/panasonic-ae4000-d65-calibration-2-lamps
A similar topic, http://www.avsforum.com/t/1472690/hw50es-vs-w1070-vs-ae4000
that might offer some more insight in the decision.