Buy the best quality foam you can afford. For the frame, go with treated hardwoods, if you can avoid using fiberboard or some kind of compressed wood product it's always better. I worked on a sectional where the body was made of melamine. It got wet and swelled up like a sponge and had to be thrown away because the frame was ruined. Basically if you spend the money for top quality materials you will enjoy your product, and get years of enjoyment from it.
Cheap furniture is usually cheap for a reason.
I'm a firm beliver that you get what you pay for. Hope this helps you out, and good luck. It sounds very exciting. I would be very grateful if anyone out there could help me out. I have found an upholsterer who has done some basic reupholstery for me which was good.