Are they're any tips for becoming a great dasher! And also, when you buy the sets of clothes, do the single items become unavailable that you purchased in the set... I don't want to end up buying the same thing twice if it all possible.

As for you first question, i've got nothing to say other than focus on chaining a LOT, even the snacks if you could. Also try to match colours as much as you can. There are walkthroughs here & if you need help with a certain level, i can try to help you (i'm not so good at writing walkthroughs myself but would be glad to try) . If you decided to try multiplayer then it's a different story. Just lemme know if you need any help in multiplayer coz it's harder than just chain & match colours. It also depends on what kind of game you're playing.
 
As for your second question, no! Unfortuantely, the things you bought will still be shown in the list.