What about lights with soft goodman handles? Are there generic sized handles that will fit any light?
I've been eying the UTD K2 LED backup light, since I can buy it here instead of paying for international shipping/customs, and it's only $70. Anyone used this light?
K2 is an older model, that's why it's so affordable, but UTD now has better offerings in their VIZ system.
I have been using a UTD VIZ20 (with hard Goodman handle) as my primary light for a year and a half now. I like it a lot. It has a dimmer which makes it great for night dives, as I mentioned above, and is bright enough for daytime use/signalling as well. It's the light you see in my avatar photo on the left.
http://utdequipment.com/product_info.php?cPath=80_95&products_id=338
The UTD VIZ10 lights are nice backups, but are not as bright as VIZ20, nor as versatile. They are not dimmable. They do, however, have the same body diameter as VIZ20, so you can use them in the same Goodman handle.
http://utdequipment.com/product_info.php?cPath=80_95&products_id=173
I have two of these stowed on my harness as backups to my VIZ20. They are probably a rough equivalent to the older K2 you mention, but more compact for the same amount of light. So far I have been very happy with the combination of these three lights, plus the hard Goodman. The VIZ20 has its own rechargeable batteries and charger, VIZ10 uses 3xAAA.
The combination of one VIZ20 and two VIZ10 will meet the gear requirements of most tech/overhead/wreck/cave courses, no canister needed. And the entire system would be roughly within your $500 budget, too.
Now the UTD VIZ 35/50 system is also really great, but lacks a sliding dimmer like the VIZ20: it only has a hi/low switch. It also needs its own dedicated Goodman because of the large barrel size. I may eventually go this route, mostly because of the ability to run multiple light heads (primary and/or video light ... and/or drysuit heater) off the same battery canister. You can also screw a battery back onto any of the heads and use them standalone.