My tractor isn't an
L3240, so feel free to use or don't use my comment... no hard feelings either way. I have and use most of the common implements... tiller, box blade, scraper blade, brush mower, backhoe, post auger, landscape rake, etc. All of them do fine for their own specific job. But hands down the most versatile, useful, hardest working attachment is the front end loader. It's a costly addition, but pays for itself many times over doing more dang stuff you'd never even think of until the situation arises. It will double the usefulness of your tractor.
3240 owners will have better comments about how big various implements should be, but it will depend somewhat on conditions. You don't have a lot of excess horsepower for its size with the 3240, which would be a consideration if you are using a brush cutter, for example, or working in hilly terrain.
Just curious if the salesman is telling you to go larger or smaller than what the owners manual recommends. Kubota usually stays pretty conservative in their printed material.