I'm not a fan of the Mighty Mouse because the scroll wheel stops working if it's even within the vicinity of dirt. I use a Microsoft laser mouse of some kind, and it works perfectly. I had a Logitech VX for a while and it worked well too (though it wore out quickly, could just be me).
Don't get the Time Capsule, whatever you do! They're still working out the bugs. I had to take mine back. So far as a 1tb external, I'd just go with something simple like
this. 1TB external that spins at 7200, and it's got firewire (400), which is way better than USB 2.0. And it's less than $200. Can you get it in the UK? I dunno, but it's worth a shot.
The iMac 24" does have 1900x1200 res. I'm confused, you said you bought an iMac, but you want to build something like an iMac?