I have a passive wine cellar in my basement. It is the closet in which the oil tanks lived before we converted to gas heat. I bought a book "How to Build a Wine Cellar" and it guided me through the process.
With the walls, ceiling and door insulated with R-30, the room maintains a year round temp of 50-55 degrees. I keep a bucket of water in the corner with a towel wicked over the side to help to evaporate the water into the air. If it didn't maintain its temperature or humidity, I would have purchased a unit that would have cooled and humidified the room that would have been mounted into the wall.
If full, it could hold about 900 bottles. I presently have about 250 bottles in it.
Is it worth it? Absolutely. It is nice to have a go to for bottles for dinner parties or when going out to a BYOB restaurant. I also have some investment/special occasion bottles.
The one thing I wouldn't be able to keep in a commercial, free-standing, refrigerated wine cellar would be the champagne that I have. The free-standing unit has to cycle and by doing so, it will shake the bottles slightly. Over time, champagne will lose its fizz if it is shaken. That would be bad.
It's really nice to be able to find a young bottle of wine that will mature with age as it sits in your cellar.
So, will your cellar be for drinking wines or aging wines? That will help determine the size and style of cellar you want.