VERY Urgent: Help me buy new wireless router for office usage.

I am a network engineer working in Mumbai (I am from South India)
I want to buy a new router for my office/home use (I live/stay at my office)
Office is total 4000 square feet with 6 rooms (including boss's glass cabin)
We will access internet from ipad, mobile etc. (very good wifi with long distance coverage preferred)
We have wired internet setup on Cable internet 4 MBPs port from pacenet
All these 3 PCs will be internally connected (networking) via this router.
we have 3 PCs now, maximum will have 5 PCs in future to connect to router (at present only 3 so you can suggest a 3 PC router too)
This router will STAY on continously for 6 days a week. (important)
I have a wired lan setup, so that this 3 PCs can talk to each other if they are connected to a router/switch from main office roon. (Need blazing fast file/transfer between this 3 PCs, important)
There are other IT offices around our office (NEED best security via this router)
None of our PCs has inbuilt WIFI (Can I get a feature in router someway which can help me connect all 3 PCs to each other wireless (without using wired cable)

We need best security + best features + best future ready router (pricing not an issue)

Please suggest best:
I found following options from flipkar site:
Asus RT-N66U Dual-Band Wireless-N900 Gigabit Router - Asus: Flipkart.com
Asus RT-AC66U 802.11ac Dual-Band Wireless-AC1750 Gigabit Router - Asus: Flipkart.com
Netgear R6300 WiFi Router - Netgear: Flipkart.com
Cisco Linksys EA4500 Dual-Band N900 Router with Gigabit and USB - Cisco Linksys: Flipkart.com

If there are any other options you can suggest me please help asap UNABLE to decide

None of our PCs has inbuilt WIFI (Can I get a feature in router someway which can help me connect all 3 PCs to each other wireless (without using wired cable)

there are wireless cards available for desktop pcs that you can use. some are usb based. others connect on the motherboard.

hi,
thanks for the information, first I want to buy a single router for my office
My question was:
Instead of connecting all my PCs with each other through wire (what we are doing at present) will there be a possiblity we can connect this PCs using the wireless router.

so please help me decide best router:
Here are some more options I have have in Mumbai:
Routers - Buy Routers Online at Best Prices in India - Network Components : Computers | Flipkart.com
wireless router online Online Store | wireless router online Shop | wireless router online Store in India Filtered on Computer and Peripherals
Buy Router in India | Buy Online Router | Wireless Router
http://www.bitfang.com/wireless-router/

hi,
thanks for the information, first I want to buy a single router for my office
My question was:
Instead of connecting all my PCs with each other through wire (what we are doing at present) will there be a possiblity we can connect this PCs using the wireless router.

so please help me decide best router:
Here are some more options I have have in Mumbai:
Routers - Buy Routers Online at Best Prices in India - Network Components : Computers | Flipkart.com
wireless router online Online Store | wireless router online Shop | wireless router online Store in India Filtered on Computer and Peripherals
Buy Router in India | Buy Online Router | Wireless Router
Wireless Routers, Compare and Buy latest Wireless Routers Online in India: bitfang.com

hi,thanks for the information, first I want to buy a single router for my officeMy question was:Instead of connecting all my PCs with each other through wire (what we are doing at present) will there be a possiblity we can connect this PCs using the wireless router. so please help me decide best router: Here are some more options I have have in Mumbai:I am not able to make links to other buying sites which are selling router.. like homeshop18, primeabgb, bitfang.so please help