Webflow's Designer is a web-based solution, however it is built as a React app which means that the designer and editor will download and run in the browser on your PC.
This is a seamless process, however to make the editing experience smooth and responsive ( important ), you will want a decent PC- anything that can efficiently use modern browser setup.
Some key points are mentioned in the Webflow forums here.
Internet connection
The faster the better. You'll notice this in particular
Storage
Webflow projects and CMS data are of course online.
Fast solid-state internal HDD is always good. If it's large enough, you can use it
Large external storage as well, I use a network-attached
If you will be creating video, assets, and working with data, you will want a decent RAID external storage solution as well to protect those assets. I use a Synology NAS in a 5TB configuration, and it's half full. Mostly video work.
