Spain in the off-season is a different world. You get the tapas bars in Madrid and the cathedrals in Seville without fighting through thousands of tourists. November and February are your best bets for lower airfare and quiet streets.
A trip like this usually runs around $2900. If you want to break that up into smaller chunks, the Citi Simplicity® Credit Card offers 0% for 18 months on purchases and balance transfers from date of account opening. This lets you spread the cost over a year and a half.
That 0% window is a deadline, not a gift. If you still have a balance once the 18 months are up, you will start paying interest on whatever remains, which can get expensive fast.