means LARGE lenses to allow as much light in as possible + LARGE imaging chip array.

This generally translates to
* lens filter diameter 70mm or larger (is better);
* 3CCD (3CMOS is acceptable) imaging chip array 1/3" or larger.

Entry point: Sony HDR FX1000 (prosumer) or Canon XHA1. (The Sony HDR-FX7 *might* be OK.)
Below this, the lenses shrink, imaging chips start to shrink and low light behavior is reduced. But the Sony NEX-VG10 and several dSLRs (in spite of their limitations) are worth a look.

All high end consumer, prosumer and pro camcorders with a lens "filter diameter" spec have some sort of add-on lens mounting method. Typically tele or wideangle (including fisheye), and if you are feeling adventurous, a DoF adapter for 35mm SLR lenses. The VG10 has an "interchangeable lens" system. So does the Canon XLH series.

All high end consumer, prosumer and pro camcorders have some sort of audio-in jack (for the external mics you will need). Consumer and prosumer cams usually have a single 1/8" (3.5mm) audio-in jack. Using XLR mics means getting a XLR adapter (see juicedLink and BeachTek, among others).

If your budget includes video lights, wireless lavaliere mics, shotgun mic, stereo mic, tripod (or maybe a vest stabilizing system or crane/jib), cases, high capacity rechargeable batteries from the camcorder manufacturer), cables, beefing up your computer to handle the high definition video for editing + the video editing software and a few other items, you should probably consider increasing your budget... A $500 mid-range consumer cam will likely not meet your requirements (specifically, low light performance), may not have audio inputs (though an external recorder like the Zoom H4 will do just fine and the Canon HF series generally have a mic jack) and for sure will not have manual audio gain control.

What have you found so far?