Here's what I will say, keep your overhead low. It seems every year there is a guy who is in his first couple years of business and goes out and buys 2 new trucks, a trailer, and some brand new mowers. Stay out of debt if possible, if not, try to keep it small. I try to keep myself from "buying from the future" aka using credit cards. I would say, right now you will need a zero turn, and trailer, and some trimmers. At least for the lawn care side of things. Start off with the lawn care, you already have some of the stuff you need for that so start there. Landscaping opens another group of tools that you need for things. What I have found while mowing is that a lot of landscaping we do ends up being for the people that we mow for.
Hope some of that helps.