Hi Nick
I'm new here too, and had the exact same idea.
I picked up a CB750 for just under £3k, given it's condition I found this to be very reasonable.
However, I did look at the CB400 (the tanks a really good shape), and these seem to be selling on eBay, at least around the £2-2.5k mark.
I've also noticed when looking for custom parts for my 750, a lot of neat looking 400 stuff comes up. So I reckon that would be a pretty good choice.
Let me know how you get on!