Thanks, Dany, for your advice.
I've seen that video and all seems quite easy, but it isn't too deep.
Ok, I understand how to modify the design (I have SiteAssistPro and know the css and template thing) but the connection to PayPal is the part that most concern to me.
Is that simple as put those "credentials" in the appropiate text fields and hit Ok?
Sorry, I ask too much because I have a client that needs a store and have a deadline to finish. I need a solution "for sure" (or better not to accept the job). Dont want to stuck in a code nightmare or too much complicated settings.