1) Yes, in fact the older the better. Older Wii's are BootMii as boot2 compatible giving you the best brick protection.
2) None that I can think of. Every firmware is able to be softmodded now, and as for hardware there isn't anything you'd really be able to check before buying it or possibly without opening it up. Things like the disc drives date and such. Just try to get one that has GameCube ports as it's more likely to be BootMii as boot2 compatible (as stated previously).
3) Play a game that requires motion+ such as Wii Sports Resort, Red Steel 2 or The Legend of Zelda Skyward Sword. If the game won't let you play, then the motion+ Defective.