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The problem you are facing is a "known hard problem" and an active area of research - sufficiently hard and active that entire branches of science are dedicated to it. The problem has a few names, and the most common ones that pop up I believe are "job performance prediction", "job analysis", and the more broad problem of "job selection" and "job placement".

The science, and thus most scientifically tested and valid tests, belong primarily to the field of Industrial/Organizational Psychology (usually just called "Work Psychology" in Europe, I believe).

As to the specific areas you want to look at, you have:

Personality and Coachability = Psychological Assessment, Personality Psychology

Aptitude = Intelligence research, one of the most studied areas in all of Psychological Assessment

Position Specific Knowledge = Job Analysis and Performance/Knowledge Evaluation (from Industrial/Organizational Psychology) + Psychological Assessment

Soft Skills = Psychological Assessment, Personality Psychology, and Industrial/Organizational Psychology

Most broadly and realistically, you are in need of help with Job Recruitment, Job Selection, and Job Assignment, which are areas of specialties in the field of Industrial and Organizational Psychology (many of whom are members of society's like SIOP). You want to be able to apply scientifically valid systems and tests to help predict future work performance. An Industrial and Organizational Psychology professional has precisely this set of experience, knowledge, and access to modern valid tests and can help you choose between them. This professional will need at least a Master's degree in their field, and probably with a preference for a PhD with consulting experience and published peer-review research in one of the mentioned fields.

Part of the complexity that requires an expert is that the most scientifically tested and validated tests are often related to modern intelligence tests, and often require a Master's technician or licensed Psychologist to apply and interpret results in a proper way, and the costs of such testing is not trivial - hours of work, a week or two likely added to your hiring decisions, and costs are likely to be in the range of $200-$4000 per person tested in this way (depending on tests, provider, etc). And it may be that that people who would be the best hires would tell you to shove it and move on to a company that doesn't require potential hires to see a psychologist first. And there's many such tests, and often a single score is not available or warranted and you'd need to know what scores are actually useful to your situation!

They aren't the only tests, though - and some tests are available in online or pencil and paper format. These are the sort of tests Wal-Mart and Pizza Hut use to screen applicants, for instance. These may or may not be utterly inappropriate to your situation. There is no general use, "here, take this and if you score well you will be a good hire" test, and what would be useful will depend heavily on it's relation to the job, business, and company.

So, much like telling a person they need to talk to a lawyer - you need to talk to a psychologist!* :) And I mean that in a good way!

*Note: In the US you can't actually call yourself, legally/officially, a psychologist unless you are a licensed clinician, so I-O psych's are often called professionals, consultants, researchers, etc, but often don't carry the title of Psychologist. But I couldn't resist the fun.