some people recommend using either channel 1 or 11 on your router..
as for getting the lan adapter, there are other compatible ones, but you need to be careful as not all of them actually are, i read before a website was selling them as fully compatible, yet only certain ones worked.
the official one would be ideal, as you would be assured it is compatible.
possibly try ebay for a better price
and in terms of affecting your laptop, anything using your internet connection at the same time will be sharing it...so if you are downloading something on the wii (wifi or lan) then your overall bandwidth available to the laptop will decrease slightly