You absolutely need insurance IMO.
Sure, I have to compete with alot of low-ballers running a $1000 8n-cutter combo working for beer money, but that is not the right way to do business, and if something bad ever happened.....they are screwed.
I have an erie policy. 2M/4M gen liability. It runs about $600/yr. Dont bother getting signatures from the customer thinking you are covering yourself without insurance. A bushhog can easily throw a rock several hundred feet. You could damage the neighbors, or a car driving down the highway.
As for job pricing, I have similar sized equipment as you. I dont charge by the hour, rather I charge by the job. But when figuring an estimate, I calculate how long I think its gonna take to arrive at my bid. Dont forget to include drive times. My target is $60/hr from the time I leave my shop til the time I return. So if a customer calls, wants 5 acres done, and it was just done last year (so I know it aint too bad), I know I can do ~2.5 acres an hour. So 2 hours on the job. If they are 45 min drive away....theres another 1.5 hrs. So 3.5 hrs x $60 = $210 So I quote them $210 but NEVER mention $60/hr. Just the flat price.
Some jobs I come in at $50/hr by the time I am, others come in at $70/hr. But it all works out in the end.
You also need to establish a minimum charge. For me its $175. Any less and it just aint worth throwing the BH on and loading up. Sure, there are alot of jobs where "someones lawnmower broke and their yard got ahead of them" and they want me to come in and do an acre or so around the house......$175 or it aint worth it. I dont get many of them jobs.....dont know what they end up doing? Finding someone working for beer money or burning up a cheap MTD rider??? Dont know, dont care.