Well, I have a decent laptop, my specs are below. It cost me around $1000-ish at the time maybe more. So that's about £630. This was about 2 years ago(or more
) and it can run all the games that you suggested mentioned(except L4d2, as I don't own it to test it, but it will probs work).
Specs:
OS Name Microsoft Windows 7 Home Premium
Version 6.1.7601 Service Pack 1 Build 7601
OS Manufacturer Microsoft Corporation
System Manufacturer Hewlett-Packard
System Model HP Pavilion dv7 Notebook PC
System Type x64-based PC
Processor Intel(R) Core(TM) i7 CPU Q 720 @ 1.60GHz, 1600 Mhz, 4 Core(s), 8 Logical Processor(s)
Time Zone Eastern Daylight Time
Installed Physical Memory (RAM) 6.00 GB