Some questions before I buy the game

If anyone could answer these questions for me I would greatly appreciate it! Thanks!

1. Is Grand Prix a story mode or just a single player fighting the CPU mode?

2. If it is story mode, is there anything to do with why people have ARMS or is it just about defeating the league?

3. Can you play against the computer or is it multiplayer only outside Grand Prix?

4. Do you think Max Brass as DLC will ruin the game (too op)?

5. As someone with a limited budget, should I buy it now or wait for a Christmas sale when more DLC is out when it is 'more worth it' (your opinion)?

Thank you so much