The upgrade kit upgrades the machine's software so that you can use the 8" x 12" (300mm x 200mm) hoop included in the kit. The machine itself has the biggest hoop as 300mm x 180mm, so it gives you a bit wider hoop with the upgrade. And the upgrade kit is FREE with their offer
If you do buy from them, ask if they will install the software upgrade on the machine for you. (It's perfectly do-able yourself, you just have to follow the step-by-step instructions very carefully.)
Which PES software do you have? They all work for all Brother machines, so if you're happy with what you have, then no need for a new one, but have a look at what they can do in case it's more than yours - they are very good prices when bought with a machine!