Hi Ruben,
You could translate the full site or only part of it (focussing on the most important parts). Only putting a translated homepage is an option but will not help you much in terms of SEO & isn't really great for user experience either.
You shouldn't worry about duplicate content if you translate the content to Spanish (and certainly don't put canonicals to the English version). In multilingual countries (like mine) it's more or less the standard approach. If you want to make it very obvious for Google (and the other search engines) you can use the hreflang tags. Check here for more info https://support.google.com/webmasters/answer/189077?hl=en or here: https://moz.com/learn/seo/hreflang-tag
Personally I wouldn't put the Spanish version on a brand new domain - as it implicates that you'll have to build your reputation almost from scratch - rather use a folder on your main domain (domain.com/es/ ) - this way the Spanish pages will benefit from the reputation of your current site. You could use the Spanish domain as vanity url & redirect it to the Spanish version
Would also add a link on each page - pointing to the version of that page in the other language or (if only partially translated) - to the homepage of the other language.
If you have questions let me know.
Dirk