The only thing you can do here is state the truth: you're not ready to accept the position at this time. You don't have to justify your decision to the employer, though if you want to keep the door open, it would be wise to say positive things about the company and the position -- which shouldn't be hard since you made the decision to apply in the first place.

You could ask how long you have to make a decision. If, as you suggest, there are not many applicants, you may be able to take a couple of weeks to explore other options and see what else there is. Mostly likely though, the company will move on immediately to their next best option, as is in their interest.