I consider my acoustic treatment absolutely necessary in my space and one of the two things that makes my space work.
I may have gone a little overboard with the amount of absorption - sometimes it's a little too dead - but I built everything (aside from the stuff mounted to the ceiling) as free standing. I can shrink or grow the "room" that the mic hears depending on where I place the panels.
Acoustic guitar? My eight 6' tall, 2' wide, 3" thick panels (that are hinged together as pairs and stand as wide Vs) are back against the walls.
Vocals? Those same panels (plus an extra 4'x3.5'x6" panel) completely surround the singer so it's dry, dry, dry.
Drums? Depends on the sound. I can move that stuff around to blend room reflections coming back into the mics.
I don't know much, but I know I didn't make too many mistakes with my acoustic treatment ideas.
Also, I'm not a carpenter by ANY stretch of the imagination. My panels don't look great but they do a hell of a job in my used-to-be-nearly-unuseable room.
If you need stuff that looks good, get GIK panels. I have two of the 244 traps above my mix position and they made my (mixing) life SO much easier. Way worth their cost if you're trying to balance time vs money. When I built my panels, I had plenty of time available for "studio build/maintenance stuff." Now, I have no time for "studio build/maintenance stuff" and only time for "music." I like that having that problem.
If I had to start over with treatment I'd send two boxes of donuts to the GIK people, send a detailed floorplan and photos of the room, and schedule a day with them to sit and hash out an approach over the phone. Then, I'd throw every dollar they asked from me at them happily.
Good luck out there!