I expect you are paying attention to the Thermalright TY-14x series and the Noctua NF-A14 and NF-A15. The two Noctuas are identical except for the frame. Between the Noctuas and the Thermalrights, the TR-140's were the quietest, most effective 140mm fans until the Noctuas came out. All of those fans act quietly, push adequate cfm's and adequate static pressure. They all achieve a great balance.
To my eyes, the Noctuas are a small evolution of the TY-14x design. The NF-A15 frames even have the same 139x150mm as the TR's. Both are superb designs.
The interesting thing about the Noctuas is that they sell you both the full frame fan with standard 140mm screw holes, and the "different" frame with 120mm screw holes. But I don't think you need an oval or oblong frame. A round one is fine, unless you do it for aerodynamic reasons. But having both allows you to use the same blade set and the same motor, conserving your investment.
PWM control is a must. You only get the cooling (and hence, the noise) you need when you need it. Otherwise, it's at idle.
I built my new rig with PWM controlled intake and mid-case fans. The PWM for those fans is set by the motherboard to respond to chipset temps. I have another intake fan (TY-140, a PWM fan) feeding cool air to my heatsink, which runs two NF-A15 PWM's. All three are running on PWM control that responds to the CPU ihs temps.
Zalman makes/made a PWM Mate. Very clever way to control PWM fans, except that putting it in/on your case is not attractive. Better: a Corsair app (similar to what you already have for your PSU's?) that reads chipset temps, CPU core temps and gpu temps. You set the app to tell fans how fast to run, and the app outputs that to USB. You could sell an inexpensive controller that plugs into a USB port, reads the app's output and tells each PWM fan what to do. (I say PWM because a fan controller using Voltage control is either expensive or can make some fans sound nasty.) With the app and the pwm controller, fans set to cool the chipset, the gpu and the cpu would all have their marching orders -- separately. And if you have -- as I do -- different fans doing a similar task (e.g. -- high static pressure case intake and high cfm mid-case) they can receive different speed control, all done in software and executed by your widget. (Speedfan does something like this now, but that app is difficult to learn how to configure, and uses the motherboard's own controller. Corsair has done better.)
No intake fan should bring unfiltered air into a case. You should sell add-on filters if you do not already do so, to complement the fans you sell.
Finally: LED's. Yes, I know purists hate them. But my daughter chose a Yate Loon 140mm fan over others because it had LED's. Sometimes pretty sells when all the competitors offer fans that are Good Enough. What you need to do to be different is to sell 120mm and 140mm LED fans with PWM control. Unique.