Go for a neutral air flow case. This means case fans blowing into as well as case fans exhausting out hot air. Most heat is generated by the CPU, Mechanical HDD's, and if present the biggie is the video card. Most video cards will act as an exhaust also. Heat naturally rises, so it makes sense for good air flow in a tower setup that has the power supply on the bottom, drawing air into the case, and vent in front and on bottom of case to have fans drawing air in. Air is then exhausted through venting in the top, and top rear of the case, in a typical ATX setup. Fans at the top of the case and rear top are oriented to push air out. Also you should notify customers that for best cooling, do not block vents, do not place near heaters or air vents. Keep several inches around the case clear for air flow. The fans used are up to you, and the number of fans also. I have a gaming case. For my input I have 2 fans in front that have air filters on them, at 140 mm in size, These work with my power supply fan also 140 mm drawing air into the case. Exhausting from the case I have a 200 mm fan on top and 1 more 140 mm rear top fan. My cpu is equipped with a 120 mm fan, vertical heatsink and exhausts through the 140 mm rear top fan. and my video card has two more fans helping to move air flow. You would think this is loud, but I run the fans at low speed, and have quiet fans mounted all places I can. The rooms heater is louder than my machine. Aluminum although more expensive is lighter and cools better than steel. Do not use Lexan (plastic) or wood, they are horrible for cooling. If they are going to be stamped metal have the edges rolled, if there are too many sharp edges and customers getting cut trying to build in the case, word will get around.