You want a guitar that plays well enough to keep you interested for years, not a cheap starter kit you will outgrow in six months. Focus on solid-top instruments that sound better as the wood ages, and look for a bundle that includes a hard case to protect your investment from humidity and bumps.
Retailers often clear out floor models in mid-summer before the back-to-school rush hits. If you find last year's model, you can often grab a higher-tier instrument for the price of a mid-range one while the shops are quiet.
The 0% intro window is a deadline, not free money; if you still have a balance once the 18 months are up, you will start paying interest on whatever remains.