I don't know a whole hell of a lot about lights, but I was just looking around on lights and I see 21W HID Light Monkey lights brand new for anywhere from 1200-2000, depending on the battery pack. It seems to me that for $465 for the light+ $200 for the new battery+shipping(Total around $700), you would be better off looking for a light in which the canister isn't beat to hell. $700 is getting pretty close to the neighborhood of a new light, especially if you can talk someone into a discount.
The way I look at buying used from someone personally it is like this.....Is the deal so good that if the light was a piece of $h!+ and you never got it working, would you be ok with that? I would think that for $700, you probably wouldn't be cool with that. I know I wouldn't.
IMHO, I would look around for a light in better condition, look for new lights on sale, or try to sweettalk a dive shop owner into giving you a discount.
Hope this was helpful.
Good Luck!