You can make a competitive spellbook with one core set. Playing well is 40% well thought out and familiar spellbook, 50% smart gameplay, 5% having all the cards, and 15% luck. I'm aware that adds to 110%, but some people only feel like winners by giving 110%.
You can't do many specific premade books (like 4 rajans fury, or 6 falcons) with just 1 core set.
You will have trouble maintaining multiple competitive spellbooks simultaneously with just the core set.
If you want more copies, get Core Spell Tomes rather than a new core set because they will probably end up cheaper in the end. I think its more valuable(and fun) to first spend money on new cards from new sets.
To play competitive, you need to be prepared for all of the released spells, because things like Intercept, [mwcard=MWSTX1CKA01]Surging Wave[/mwcard] [mwcard=DNA01]Acid Ball[/mwcard], or [mwcard=DNQ07]Veteran's Belt[/mwcard] can mess up your day.