i'd say,
consider who's giving you advice on what to consider, before considering it.
then, consider what a user and how many potential users, would do with the domain before buying it.
don't worry about how many letters, because if the domain has 20 something letters and three words, it may speak to wider audience, than a shorter vague name with no clear meaning.
consider applying as much common sense to your selections.
How to come whether the domain is spam free or not, any tool for this, or may be blacklisted by Google for certain activities, as somehow any person who have website, must consider to do SEO ,,,