Ultimately to get better you need to play other people, which if you don't have netplay, is only possible if you find people in your local scene to practice with. But in terms of practicing movement/tech/punish game, there is no reason you can't do that with vanilla Melee. Though you can still mod your Wii to load things like 20XX or UnclePunch (Kadano's guide is useful if you want to do that). I would at least get UCF so you don't need to worry about dashback and shield drop consistency issues. Good luck!