Cheap approach is 2”x4” (or country specific equivalent) with 1” plywood shelves. Because of weight, you have to design and brace the corners very well or it will “rack” and collapse.

Next cheapest solution is a wire frame shelving unit with multiple adjustable shelves.

Conduit + pull string.

If you need to add a cable, add it the existing pull string along with another pull string. This allows you to have another pull string in place, since you will have “used” the existing pull string.