Well, what purpose is the disorder playing in your story?
Like, if it’s the disorder that has brought your characters together, which aspect of it is important to them/affects them most? How much do you WANT it to affect them? In H/C we aren’t writing for the illness/injury, we’re writing for how that affects our world and characters. You can totally control how bad shit gets for them based on what you want to have happen to the characters.
Example: One of my very favorite OCs has COPD. At times he has exacerbations which seriously affect his ability to do much other than lie propped up on pillows, snuff oxygen, and curse the universe. At times he’s got his symptoms under control with medication and rescue inhalers. His condition makes him particularly vulnerable to certain situations. His experience with this condition makes him very aware of what needs to be done when someone else is experiencing similar effects. Because of his experience with this condition, this character is pretty well used to physical discomfort and some extent of disability. He knows his limits and exactly how far he can push them and what he will experience if he pushes past them.
A chronic condition, like the one I just described, is a major, sometimes overwhelming, influence on a character’s whole life. Everything they do is necessarily colored by the effects of that condition. However, if you want to throw something recurring but asymptomatic in periods of remission into the fray, you get a whole fuckload more freedom in what happens in your story. Christie uses malaria more than once for its useful periodic nature.
Acute conditions really require you to do the research. Look up how first-responders assess and triage patients. Airway, breathing, circulation. In your standard-issue country-house murder, Lord Victim is either minus all three or in a stage of acute distress due to ingestion of arsenic or cyanide. Heart attacks do not always present with chest and left-arm pain, and in women heart-attack pain can be referred to a number of totally odd places including the back, neck, jaw, and stomach.
Think WHY you want the character to have this condition, what plot-related importance it has, and then work out from there how severe it needs to be in order to get the effect you’re aiming for.