With regards the different models listed in the first post, anyone have suggestions?
Given those set of choices I'd probably get the refurb 24" unless you think that screen will be too big for you (believe it or not, I've heard people complain about that--depends on the layout of your desk and how far back from the computer you can get).
At least in the US (I assume UK is the same) Apple's refurbs are generally quite nice--they're most often "open box" returns, meaning in mint condition. Since it comes with a full warranty, from your point of view the only difference is that it probably comes in a slightly less attractive box. If it's whatever the previous iteration of that model is, you should of course be aware of anything that's changed (processor speed, chipset, etc), but if it's the current gen it's basically new but cheaper.
I've recommended Apple refurbs to a number of people, and the only issue that's ever cropped up was a bad optical drive in a Powerbook, but that was fixed under warranty, and of course you can have the same thing happen on a brand new machine.
If you don't want a refurb, it's probably in your best interest to hold out until the next update, since you don't seem to be in any hurry--at that point you can chose between a recently-refreshed iMac or the previous generation at clearance prices (at least in the US, Apple usually lists the previous generation models in the refurb section at significantly cheaper prices until they've sold out, and other retailers usually do the same).
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