1) Practicality of sofa. In Singapore hot and humid weather, breathable fabric made into slip-on may work better, since it can be machine wash. You do not have to worry about the long term usage of it.
3) If it is custom made, you should have the option to select the exact cushioning density, be it firmer or softer, before committing to it.
4) Designs and fabrics should fit into the space. A home with plenty of whites and sleek lines may do well with a fabric that is tastefully louder in its patterns. Whereas, a crowded room, should use fabrics that is less busy. Always keep in mind the end result you want.
5) Go direct to the manufacturer, because they cut cost from not having retail stores. You will get better materials for the same cost you put in. So buy smart and go straight to the manufacturers.