If you have a DC, you should own this game. It's fairly common, so you could probably find it for the same price, complete (original case and manual). However, if you don't care about that sort of thing, and just want the game, then go for it. Otherwise, I'd look for a complete copy.
Cool, glad you're enjoying it. Considering the age of this game, it's surprising how well it's held up over the years. I guess you shouldn't expect anything less, from one of the greatest fighting games of all time!
$13 sounds like a solid price for both games, especially if they're complete.