Building a silent HTPC with Asus Z97-A and i7-4790k in a hot country

for an HTPC, you DO NOT need the i7. if you would like to play games on it too, (unless you have a 4k TV) I would recommend getting a i5 (no need for a k sku, even the i5 4460 is fine) and a GTX 960 with a nice after market cooler on it, all that inside a tiny mini ITX box. I've got a list for you, so you can understand what your really need:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor ($176.95 @ SuperBiiz)
Motherboard: Asus H97I-PLUS Mini ITX LGA1150 Motherboard ($101.99 @ SuperBiiz)
Memory: A-Data XPG V1.0 8GB (2 x 4GB) DDR3-1600 Memory ($42.99 @ Newegg)
Storage: Western Digital Black2 1TB 2.5" 5400RPM Hybrid Internal Hard Drive ($139.99 @ Newegg)
Video Card: MSI GeForce GTX 960 2GB Video Card ($198.99 @ SuperBiiz)
Case: Silverstone RVZ01B Mini ITX Desktop Case ($79.99 @ Directron)
Power Supply: Silverstone Strider Gold 450W 80+ Gold Certified Fully-Modular SFX Power Supply ($88.99 @ SuperBiiz)
Total: $829.89
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-07-26 10:51 EDT-0400

I am personally a fan of having a bit of air flow in a system(If you happen to have a slim optical drive chances are it will make more noise than a quiet fan anyway, but some are more quiet by slowing down on DVD/CD/BD, but still loud with data discs), but that passive case could work. This would make the Silverstone an interesting option to me(but it will not be as quiet as the other one).

The onboard video should be upto the task of watching video(Bluray/DVD/Streaming/ECT), but even something like a simple 750 or 750 ti could allow some med/high detail gaming(1920 x 1080).

About the power supply.
The HDplex is a DC-DC power supply and does not have to deal with the 120/240 AC to 12 volt conversion and instead deals with I think 16-18 volts down to 12/5/3.3/ect. This means you have a large power brick outside the system like an Xbox 360 video game console. That is how the power supply gets to be soo small.

It is not exactly super powerful because it is rated at 192 watts @ 12 volts with peaks of 228. This is the point that a slightly slower cpu would have come in(I know you have one anyway). This rather low 12 volt power rail dictates what other hardware should be used in the system. Notebook drives and SSD's are best for the lower power consumption(and 2.5 inch drives tend to be more quiet than 3.5 inch).

I would build a system without a video card and see how it works. A video card can always be added later. The passive case may need optional components to passive cool a video card. Streamcom also has its own passive power supply(rated a bit under the HDplex one).

Sorry to not have exact specs, but every user has different requirements. My media center PC uses weaker hardware(i5 750, gtx 650ti[my cpu has no onboard video], 3TB WDC red[storage], 1TB WDC Scorpio Blue[Media center PVR], M4 128 SSD[OS/Programs], Slot load Sony optical drive and external Bluray reader). It is more than upto the media playback duties and some light gaming. It uses an SFX 300 watt power supply(not the most quiet under load, but fine for me with a swapped fan). The system is inside of a SG05 from Silverstone(A perfect size for my use).