Adding a humidifier to a heat pump?

[Ruby the Liberal]

2. My heat pump unit is in an upstairs closet

that was almost built around it.

Do they take up a lot of room? Is this something a handyman can do, or should an HVAC be called?

Also, if you know - how do they work? In googling I see references to humidifier controls (like temperature controls) to ensure that there isn't too much moisture as it can lead to mold issues and an increase in a potential dustmite problem. Is this something that can be combined with a thermostat, or do they have to fish the wires through the wall for a separate control?

Since I have you - are there options, and if so, what to look for/ask for, and more importantly - what corners shouldn't be cut for an ill thought cost-saving desire?

Just trying to understand this so I know who I should call and if what they are telling me gels with what people who understand this have advised. An elderly relative suggested this to me this week but couldn't speak to the particulars.

I don't want to pay $1,000 for something that can be accomplished for $500, but in the same token, I don't want to pay $500 for something that, done right, should cost $1,000. If that makes sense?