Getting residence in Spain while working part-time

As a British Citizen, when applying for the EU residence certificate as a part-time employee, is there a minimum number of hours of work per week you need to get it?

What about if you had a contract for just a few hours per week, earning only a few hundred euros per month? Could there be any problems?

Would you have to also prove you have savings to live off - and thus go down the other route of bringing in Spanish bank statements / possibly private health insurance too?

And what about gaining access to Spanish state healthcare? Would working just a few hours per week give you access to that?