You wont get an Apple for £400 (unless you go for a 3 year old+ model) so that means you are stuck with a PC. As for Vista, I was a big skeptic over upgrading to it, but to b honest it is now a stable and good performing OS, sure it seems a bit bloated compared to XP, but hey XP seemed bloaty compared to 2000, and likewise 2000 to windows 98/95 But I dont see many people harking on about wanting to install that on their PC's as it was so much smaller.
I wouldn't however wait for the new windows, the problem with operating systems is when they are new, by the time the OS is 12mnths old on the whole the majority of bugs and problems have been fixed and patched out, hence why Vista is now an acceptable as an OS.
As for which model, I dont know, I am not up to date with whats hot or not in the entry level laptop market. However for web browsing & other basic functions I dont think there would be anything on the market that would not be able to handle it.
With regards to an external modem, are you refering to dial up, If so I would highly recomend going for broadband, it makes connecting to the net so much simpler. Im sure all Laptops these days have built in wireless connection so all you would need is a wireless router (often provided by whoever you chose to provide the broadband). If you insist on staying on dial up, I am pretty sure most laptops will come with a built in modems, although I can see this fading from basic specifications as the technology is pretty much obsoleted in these days of high speed wireless/wired network connections.
Ordinarily I would say dont buy from PC world (as they actually know very very little about the products they sell, and most the stuff can be bought cheaper from other outlets) but to a novice there is the advantage that they are a "high street" store and they can easily get in touch if things go wrong.
(written on my 5 year old Apple Ibook ...its old but does everything I require of it...My desktop handles the more complex stuff, this is simply my sofa surfer/email reading machine)