Just a few thoughts. I've had a home darkroom since 1996. I built things capable of printing up to 16x20, but have never exceeded 11x14. B/W only.
Consider some way to filter air coming in when you ventilate your space. Dust is your enemy on drying film and inside your enlarger. Most bathroom fan types suck air out of the room. It's easier to filter air if your fan blows air in and you filter it at that point. Filters are a good thing. I have an air filter in the darkroom (circulates air through the filter), but not on the air intake (my fan is the suck air out type bathroom fan).
Paint your walls white or something light. If your safelight is safe and your room is light tight, you don't need black walls to make it darker. Assuming this is a single user darkroom, you probably don't even need dark walls around your elnarger. But your enlarger is the main source of non-safe light you're going to have.
Your enlarger mounting -- table or wall mount -- needs to be absolutely stable. If a door, especially an exterior door (heavy) is anywhere nearby, don't mount the enlarger to a wall, it will shake when the door closes. My DR is in a basement and I chose a table on the cement floor. I built a simple, but strongly braced table out of 2x4s with a particle board top. If I ever have to move, it won't get through the door without taking it apart.
A floor drain is a great thing if you can swing that! I don't have one. I wet mop the floor every couple of years.
You will have to choose a max print size. Everything scales up as your prints get bigger. Be realistic, but don't keep things so small that you'll want to rebuild before too long. Hard to predict the future. The bigger everything is, the more difficult it is to deal with. Counter space, chemical volume, volume of space to keep dust free and so on.
Buy stuff used. There are a lot more people getting out of wet darkrooms than getting in these days.
Assuming you'll be using open trays for processing, remember a darkroom is going to be almost as humid as a bathroom. Consider that in your choice of construction materials. You might want to store unused paper outside the humid room and you might want your print drying structure outside the humidity also.
