I'd buy it if I had the cash. Gauntlet Legends is a great game, and it's truly meant to be an arcade game. If you're just looking at it for conversion purposes, though... well, it's still a good deal. However, the civilized thing to do is to keep it running Gauntlet Legends. 😉 I know you said you want to convert it without gutting it, but I assume you're still going to take the board out.
edit - oh yeah, doesn't the game use analog sticks or 48-ways or something? You'd probably want to replace those if you were going to use it for MAME.