Hi. The shed would need to be pretty big to accommodate a band practice. Mine’s 10′x16′ and that is probably on the tight side. Definately do a self build if you’re a practical person, works out cheaper for a better shed. I’m guessing sound proofing won’t come cheap, but I reckon you can get by with lots of layers of carpet on the ground (will keep yer feet warm too), and probably loads of sponge on the walls. I guess the most important thing is avoiding ‘hard’ reflective surfaces that bounce the sound about maybe. I’m sure you could find forums about it. Or maybe insulate the walls really well then cover them with some kind of carpet to help diffuse the sound waves. 2 doors (as in one behind another, not double doors) also really helps. Hope that’s of use?!