My budget is around £1000-£1500 for the computer and monitor - and I have a friend that can probably get me a 17% discount on Apple stuff.
The Apple Refurb store is your friend:
http://store.apple.com/uk/product/FB470B/A?mco=MjE0NDk5Mw15" MacBook Pro for a hair over £1K. With £500 left in your budget, you could easily spring for a (non-Apple) display, although if you already have a monitor, you could use the current monitor as the main display and the MBP as the secondary one for palettes 'n' stuff. If you already have a full size keyboard and mouse, you can hook those up too.
The laptop and monitor idea seems cool, but as I'm not at all concerned about portability, I might be paying for a second screen I'm not using.
Ah. You say that
now, but I absolutely guarantee that as soon as you start working with a laptop, you'll be taking it out into the garden, working from the living room/ bedroom/ toilet and even -- as I have taken to doing if working from home is sending me a little stir-crazy -- decamping to the pub and spending an afternoon working there.
It's also worth noting that if you have never used an Apple laptop keyboard and trackpad, it's impossible to appreciate how many orders of magnitude better they are than the nearest competitor.
I want as big a monitor as possible (and I want to use my existing monitor as a second screen for all the tabs etc), but I greatly dislike those shiny ones that Apple are doing now.
Ah ... then you're not going to want an iMac then. That leaves you with the Mac Mini, a machine I own and love with a passion but which is certainly underpowered for what you want to do, a laptop with an exertnal monitor, or a MacPro, the cheapest of which will blow your budget by £400 before you've bought a monitor!
Where -- you may ask -- is the mid-range Apple tower? Head over to Ars Technica and search up any of the thousands of posts on this very subject -- AKA the xMac -- and you'll see that this is a somewhat contentious issue. Bottom line: there ain't one, I'm afraid!
I'm going to use the machine primarily for Photoshop/Illustrator, but am also thinking about running Windows on it, and using some 3d programs like ZBrush.
Not done it myself, but I believe the MBPros have been road-tested as Windows laptops and perform very creditably. The model I linked to above outspecs my iMac quite comfortably, and the iMac deals with Cinema 4D and Z•Brush fairly capably.
Cheers!
Jim