It should be enough, but the rules are never 100% clear (surprise) and can even vary from one region to another. For more on the proverbial "how long is a piece of (Spanish) string, please pop in to the "just curious" thread.
To get a "pardon", you need to behave very well ; to get on the "padron", which is like the local census, strictly speaking you should be resident, so you may or may not be able to sign on it anyway. Who are you buying the car from? If from a dealer, they should handle the transfer & registration for you. If you're buying privately, you may want to use a "gestor" to handle it all for you.