Sounds like you've thought this out somewhat ... a 36" WB for the smaller areas and gated yards ... a Z that's not too big, but big enough to tackle most any residential or small commercial property, blower and trimmer.
As for a trailer, I would look for a good used 16' or so, tandem axle. You can get by with a smaller one, but you're probably gonna grow and the extra room now would be nice. You'll probably add a 21" in there somewhere, various small hand tools, gas cans, trash barrel maybe, etc. Try not to go with something too small that you will outgrow this season.
Be positive, aggressive, and treat your customers well. Image means a lot in this business, as that is part of what we are providing our customers. Sure, you'll have some who all they want is the dang grass cut, but you'll have many who will appreciate your efforts to make their lawn look nice. Many people want their lawn to look better then the neighbors, and they look to you to do that for them.
Good Luck ... and keep reading here on Lawnsite ... there are many great people here!
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