Old Wii system and Skyward Sword?!?!?

First up, you will need a Wii Motion Plus add-on OR a Wiimote Plus to play the game... as that's a requirement, so you may need to buy one if you don't have a Wiimote Plus OR Wii Motion Plus already.

Having that said, I will note that Skyward Sword does include Wii Menu Update 4.3 on the disc itself... so your "Old" Wii will get a firmware update to 4.3, if you don't have it already (via online update). This way, your Wii will have all the IOS's required for the game. Firmware updates are always free.

Don't be surprised if you see "Wii Menu Update" in the Disc Channel (Instead of Zelda: Skyward sword)... as it just means that you have to install the firmware updates before you can play the game. It'll take a couple of minutes to install the updates (depending on how far it has to update from... so it may take longer if it has to update from Wii Menu 2.0 than 4.0 or 4.2)

I will note that my Wii (purchased May 28, 2008) is running the game just fine... so don't worry to much about it.

Hope this helps!

Video game consoles are not like computers: all Wii games are compatible with all Wiis. You will, however, need the Wii Motion+ attachment, as you said. If you buy Skyward Sword new, it should come with it, if not, it shouldn't be too difficult to find.