Kayaking, especially if you live in Denver, is a two-boat sport. You'll want to have a playboat and a river-running/creeking boat.
Get a playboat first, since it's almost Winter and you'll want it for the pool or Golden PP. I'd suggest getting something that people have heard of and has a good reputation (Kingpin, Allstar/Fun, or Project/EZG). There's probably a reason why nobody chimed in on the Supersonic. It can be difficult to tell the differences between boats by just looking at them and similar looking boats can paddle and roll very differently.
Then, save up some cash and get Dan's H3 to run rivers and creeks in the spring. The H3 is fast, stable, and rolls easily. It would be perfect for Lower Clear Creek or the Ark.
In my opinion, this just isn't a great time of year to buy a used kayak. Lots more used boats will pop-up as Spring approaches. You could also ask around at pool sessions and demo boats at the pool this Winter.
If you're set on getting just one boat, I'd recommend a downriver playboat, like the Fun, EZG, or Fuse.