general liability insurance is not required by law, but someone operating without it is completely stupid and is not only putting themselves at great risk, but they are putting their customers and the public at whole in extreme risk. If you want, do a search of my posts about insurance. I have told stories of guys I know and different situations in which they had no insurance. I am glad to see you plan to be responsible and get insurance before starting.
You will need commercial auto insurance. Massachusetts law says that if you are using a vehicle for commercial purposes (ie doing landscaping) you must have commercial plates and commercial coverage. If you operate with regular personal plates, registration, insurance, you are essentially operating an improperly registered and improperly insured vehicle. The law considers an improperly registered/insured vehicle the same as operating an unregistered/uninsured vehicle. I have told the story on here of a guy I know who got in an accident caused by someone else, he was driving a personally registered truck and the officer noted in the accident report he was using it for landscaping. Long story short, insurance refused to cover it, he was sued for hundreds of thousands of dollars in damage and lost everything, his wife and him are on probation with suspended sentences until it is paid off.
As far as what to spend your money on, that is too hard to say. Everybody's business is going to be different. Some guys would use it for an older truck and equipment. I would get a loan for a newer more reliable truck, newer equipment, and save that cash for marketing and for capitol until the business takes off. I don't think I saw it anywhere, but will you be doing this as your only income, or will you be keeping another job until the landscaping takes off? That will play a big role in how to use those funds.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!