Most people building V1/MPCNC machines use Marlin firmware. Easel is a basic tool that output extremely simple g-code. Easel was likely intended as a GRBL tool, but, since it uses just a handful of g-code commands (when I tested it a couple of years ago), the files run fine on Marlin/MPCNC. See this post. Given the basic nature of the tool and the g-code, I remember seeing a couple of posts on this forum where a script was used to add functionality.
As for what board to buy, any board that runs Marlin and supports five stepper drivers will work to drive the MPCNC, but some boards are potentially easier to get running. Even if you don’t buy from V1, the path of least resistance is to buy an SKR Pro 1.2 board or a Rambo 1.4 board. The firmware for these two boards will have the most testing, and, if you purchase from V1, they will come pre-flashed with the firmware. If cost is a significant issue, you can go with a Ramps 1.4 board. The Ramps board is just as capable in terms of cutting, but it tends to be more delicate with a higher percentage arriving DOA.
If you are considering a board beyond these three, or if you are considering a GRBL board, I would suggest you run the board by this forum and get some feedback.
Edit: Just before closing my window I snagged on the phrase “interested in loading easel.” Easel does not go on the control board. It is a browser app that has the ability to save g-code to a local file. Easel is just one tool (of many) that author’s paths and generates g-code that is then run by Marlin. The process is similar to 3D printing where multiple tools and slicers can produce g-code that is executed by the 3D printer. See the Software Workflow page.