My first projector was the AE1000 (the bigger brother to your AE100), which unfortunately got damaged when it was knocked off the temporary table stand we were using, so it was replaced with the newer AE2000. I've upgraded a few times since then as I was never really happy with the contrast of the Panasonics; I went AE1000>AE2000>AE3000, then JVC HD350 (big jump IMHO), JVC X35 and now JVC X500, with a worthwhile (to me) improvement in picture quality with each step.
Most of those would be in your price range used, though with various caveats (so do some research before committing to buy ; ) ). I haven't seen the cheaper Epsons that you can buy new for not much above your budget, but I would be surprised if they weren't much better in terms of contrast and general picture quality than your AE100, simply because the technology has moved on and the 'trickle down' effect has occurred.
They are likely to be much brighter too, yet will still have a lower black floor (due to improved contrast), so you'll get a punchier image, but with better blacks than you have now. This of course depends on whether you can black out your room and the type of decor it has: Putting a £10k JVC in a white room with thin curtains and lots of windows/ambient light isn't going to be much improvement for example, but provided you have a reasonably dark room you will at least see the benefit in the lower black floor and higher resolution.