A properly configured iMac is a great machine for video editing. Anyone that tells you that a PC is the only thing for heavy video editing is giving you bad advice. Across the world, full-time video editors are happily using the iMac to make a living, including myself. Don't get me wrong, there's nothing wrong with a PC...I'm just saying that both a PC and an iMac can do the job well.
As for what you need, if you go with a regular iMac, you will want to get an i7. With a regular iMac, get one with the base-level 8 GB of RAM and then upgrade the RAM yourself to either 24 or 32 GB. (Super easy to do.). I strongly recommend getting the internal SSD drive. 1 TB is probably preferred, but 512 GB is workable depending on the size of the projects you are working on. An i7 iMac is perfectly capable of smoothly handling most 4K workflows.
If you will be cutting and editing really high bit-rate 4K video (such as the Canon 1D X Mark II in 4K/60 fps) and you want a highly fluid experience, then you may want to consider the iMac Pro. If you are cutting and editing 8K video, then you might want to consider an iMac Pro. If inaudible fan noise and silent acoustics are very important to you, then you might want to consider an iMac Pro. But if you are simply editing 1080p video or 4K GoPro video, then an iMac Pro is total overkill for your needs. For the latter, the mid-2017 iMac i7 will serve you well for several years.
Hope this helps. Feel free to ask away about any of the other configuration options. There are a lot of great people here that can offer up some advice.