20 hours a week is considered full-time, but you have to make sure that the school you choose can give you the proper official paperwork that the consulate needs to process your student visa application (not all schools can).
I'm not sure what consulate you'll be going to in the US... but it really is hard to get info over the phone. Go in person, if it's feasible. If not, try calling the main embassy/consulate in DC.
Once in Spain, if you get a work offer from a company that is willing to make you legal (not an easy feat, honestly), then you would have to go back to the US and present your paperwork at the consolate. They will hold onto your passport while they are processing it, so you'll have to plan to stay in the US during this period (usually two weeks, depending on the consulate).
Remember, a visa is only a document for entering the country. Once you are here, you will need to apply for residency (with a student visa, you apply for student residency; with a work visa, you get a work permit).