Linux can be a real PITA at times, I'd say stick with Windows if thats what your used to
My brother is fairly senior in the IT dept of an insurance group (which has just been taken over by an even larger group, so he's waiting to see what happens!), and so has all the latest kit, etc. He's tried Linux, etc, and although he likes a challenge, even he couldn't be bothered with it much in the end. If Windows (and its going to be Windows 8 these days) works for you, keep it simple. If you want to enter the brave world of Linux, etc, you can always dual boot your shiny new laptop later!
I agree that £300 is going to be tight for a 15.6in screen with a 1TB harddrive, with a powerful processor. I dont really do the computers where I work, since I'm really TV's, but since the laptop I'm using has seen better days, I've had a quick chat with the guys on that section. Their advice is not to worry too much about the exact spped of the chip, but as others of mentioned, get the biggest HDD and most RAM for your money. If your not too bothered about AMD chips, then this might be worth a look:
http://www.johnlewis.com/lenovo-g50-45-laptop-amd-a8-8gb-ram-1tb-15-6-black/p1759617
Although you can go second hand, its worth looking at what we call Branch Mark Down's. In the above store they will have a red ticket (normally saying 'reduced to clear', which generally means they've come back from a customer for various reasons, or 'ex display', which speaks for itself). Since people return laptops for all sorts of reasons (I hated the keyboard on mine), you might get a good deal, since they've all got full two year warrenties. Summer Clearance is coming up too.
Remember that laptops get renewed on a three month cycle - there are four 'new models' a year, so there is a bit of churn. That means that an end of line machine might not be that old.
As for software, the basic MS Office (I think) still comes with machines, I often use Open Office on my older one. On the other hand, if your office is using Excel as standard, your going to need to get something which works with it fine. I'm sure someone can steer you to a cheapish version!
As for where you buy it from, thats up to you. I have to admit that buying computers online has not always worked for me. Its really worth touching the machine, just to try out the keyboard, etc. If its awkward in the store, its not going to get any better at home. And get an external HDD to back up everything ASAP. They are really cheap now (500gb for £35), although you can also use the Cloud.