Any Intel Mac is going to do what you want and more. I own five Macs. The only one I wouldn't want to do video editing on is the G4 iBook. Even my 1.83 Mac Mini works great for video editing. Typically, when editing video, the graphics card is the least of your concerns. The bottleneck in your workflow will be 1) capturing your video, which you can do nothing about since it has to be real-time, 2) rendering, which is dependent on processor speed and RAM.
If you end up with the iMac, get the 24". Not just for screen size but screen quality. The 20" uses a TFT screen which is only good if viewed straight on. The 24" uses S-IPS which is much better for off-angle viewing.
But, for $2500, you can get a Mac Pro and it doesn't get any better than that. You can load it up with 32GB of RAM and four PCI Express expansion cards. It's user serviceable unlike the iMac or Mini. However, you have to buy all the peripherals which will put you over the $2500 mark.
Used Macs hold their value so you won't save much with them. However, I highly recommend the Apple refurbs. The last three Macs I bought were refurbs and they were indistinguishable from new.
Finally, pay for the AppleCare. 3 years of piece of mind. And in 3 years, your Mac will still be very usable unlike cheaply made Dells and HPs.