Assuming that you are using final cut pro or iMovie, Apple does a really good job of optimizing their software for each of their computers. I would suggest the mid-spec 21.5 inch 2018 iMac with the storage option set to fusion drive. I would recommend this because it has a quad-core i5 and 2gb gddr5 graphics memory so you will be able to edit videos pretty well without leaving you bankrupt. I would suggest you upgrade to a fusion drive because the standard hard drive is very slow for a new computer. The Fusion drive combines a hard drive and an ssd so you will have quick boot times but a lot of storage for your video projects. As for memory, you can leave it stock with 8gb, but 16gb will last you longer and is really a better deal.
If you are looking to upgrade your current iMac, then unfortunately you can't upgrade the graphics performance, ram, and CPU since those are part of the logic board.