why not start with a used walkbehind mower like a bobcat or scag roughly 32 to 36" and add a sulky to it that way you can ride around the yards. You can find good condition belt driven bobcat 36" wb's for good deals. I have seen them on craigslist selling for 500 dollars and just needed 1 or 2 replacement belts. With proper engine care they can last a forever with just replacing belts. Sounds like you have solid trimmers and a blower but I would add a push blower to your arsenal for leaves and debris.
I have a 4x8 and 5x8 trailer both open trailers. I am going to convert the 4x8 into a leaf trailer for the fall and also use that for mulch jobs during summer and spring. I would go with an open trailer at first until you know as a fact that your going to get enough buisness.
Also, starting with the walk behind you will be able to guage if your going to have large enough lawns where you would need a riding ztr. These are just my opinions.
2006 Honda CR-V
08 Yamaha Wolverine w/ Warn Winch and Plow
5x8 Trailer With Folding Gate and WeedWhacker Racks
4x8 Trailer Converted to Leaf Box Trailer
96 Bobcat 36" WB w/ Dual Wheel Sulky
21" Yardman Push Mulch/Bagging Mower
Toro BrushMaster Weed Eater
Echo PB500H Blower
Troy Bilt 5hp Push Blower
Ryobi Elec. Start Blower- It works and works damn well
Echo Gas Hedge Trimmer