Which wireless router do i use if i already have a ADSL modem provided by my ISP?

The best option is to replace the Existing modem provided by your ISP with a combined wireless router/modem. The reason for the same is that it will difficult to find a wireless router compatible with your ADSL Modem.

What is NAS and which Routers can I connect NAS?

NAS is Network Attached Storage. In NAS a Hard Disk is set up with its own network Address, its not attached to directly to any computer which is running applications or files for any Network User. It helps is serving applications and files faster than the conventional way as they are not using the same processor resources.

NAS is attached to a LAN or local Area Network and assigned an IP address.

NAS includes of hard disk storage which has multi-disk RAID systems and a software for configuring & mapping file locations to the network attached device

Also NAS can be made into a more sophisticated storage system called as Storage Area Network or SAN

Routers which are compatible with and fast at quick and speedy Data Transfer across networks.\

Linksys WRT350N and WRT610N

Linksys WRT350N

What hardware do i need to connect Two Wireless PC's and a Wireless Laptop to my Broadband Wirelessly?

What you need is a 4 port Wireless Router, this way your can connect the PC's via Ethernet and the Laptop Wirelessly . If you need your PC's to also connect Wirelessly you need USB adapters or PCI cards on your PC's.

What I can recommend is the the Netgear DG834N and for USB adapters Netgear WN111 Wireless-N USB adapter.