When time and budget are a problem, it is most efficient to create two websites and exchange the text. With a little more effort, you can use a voice button and
Creating a site that uses both languages on the same page costs more design time because you need to design about twice as much content. Besides, you will repeat yourself a lot, and the English alienate the Spanish readers and vice versa.
And if you want more languages in the future? These tiny medical brochures come to my mind.
Also keep in mind that if you start with a single site and divide it later, you'll throw away all the work on that layout, because it feels different with half of the content (and probably far too empty). So that's a huge waste of resources.
Include multiple sites (be it across different domains, subdomains or a button or cookie or whatever) as a good start for localized information. Not just translations, but actual content, for example; Daily newspaper introducing new products on different dates for local / international customers.