Mario Kart Wii is (while still a decent game), generally considered to by far be the most mediocre entry in the series. I would instead recommend buying a cheap Wii U for Mario Kart 8, which is a much better game, and generally considered to be either the best or second best Mario Kart game - depending on who you ask.

If your goal is to play online, then Mario Kart Wii won’t help you at al. The servers are dead. You literally can’t play it online anymore. If you want to play Mario Kart online, your best bet would be the Switch version of Mario Kart 8. The Wii U version likely still has some activity, but finding a match will in all likelihood be a long wait most of the time.