if it's in good condition, then $300 is a good price. all of the current CTS kits (e.g., bicycle, ski, jogging) should work with a 2005 Chariot
AFAIK. There have been changes to design (e.g., they removed the glow-in-the-dark coating from the top cover in favor of strategic Scotch-brite because, in part, the glow-in-the-dark comes off in the rain, lol)---however, these new parts are also widely interchangeable (of course, check directly with Chariot for your particular model---they're normally very quick to respond via email). In one instance, i replaced my 2005 top cover with a new (then) 2010 cover; it was a perfect fit.
in short, the changes and differences are small; all of the major systems are identical. a well-priced in-good-shape used Chariot will save you a lot of coin over a brand new Chariot, and will give you 99% of the benefits.