So in the US at least, in most states/cities you would need a license to touch a patient. With a modality such as energy healing you don't need a license because there is no physical contact, but with acupressure there is. That said, there isn't a general acupressure license - but most of the time it would fall under massage therapy. So to legally work on others (laws would vary by country/state/city) you would need to obtain a massage license. If your interest is in Chinese Medicine theory you would want to go to a school that has a Shiatsu (Japanese acupressure/massage) or Tuina (Chinese Medical Massage) program or at least adjunctive classes.
Now, with regards to diagnosis, with just straight acupressure you don't need as clear of a diagnosis or any at all in many cases because you are simply using points for specific conditions/issues - LI 4 for headaches for example. With more complete systems such as Shiatsu, you would use abdominal diagnosis, possibly pulse diagnosis depending on how you were trained and then design a treatment which involves working on certain meridians with specific point use that is nearer to some of the complexity with acupuncture, but still close to the basic'ness of working on a point -to- condition paradigm as in most acupressure uses.
Acupuncture is more complicated largely because you can stimulate more than two areas at once (more needles than hands, of course). This means that there is a synergy with multiple points of stimulation that is more powerful but also more complex - hence more training in the diagnostic aspects.
In my opinion a good mix for someone really interested in helping people and using bodywork to do it, would be to study shiatsu and then later craniosacral therapy and/or energy healing systems such as tong ren therapy (how to learn) and go from there. You can accomplish quite a bit that way.