Depends on the shape it's in. Anywhere from $5,000 to $7,000 I'd guess. Look on KBB.com and check out the purchase price. It at least gives you a starting point. Check out ebay and craig's list to see what people are selling other ones for. Summer sales are always better for the buyer, since there is no imeadiate use, so you can get it a little cheaper. Fall will be more expensive, as more buyers are out there.
The IQ Chasis is a rider forward style, but not as dramatic as the Doo's have. It's between the old style sleds and the Doo. But you can change that a little with an articulating riser bar, and force yourself more forward or back. I think you'll find it a good setup, and ski preasure won't be a problem. The seats are tall, so you have a comfortable leg position while sitting, and you can actually slide back and stretch out if you want to. Not sure you can do that with the Doo's. I haven't ridden one long enough to try.
Good luck. Hope it works out for you. Regardless, any new sled is going to be great for you.