There are many ways to do this and succeed. Also many ways to fail, and those ways are very similar to the ways you succeed. In other words, high risk.
This is not scientific, by any means, but I'll throw a little out there.
If you buy a new truck and trailer, I would have $10k on hand to cover fuel expenses as well as good credit to get started. You could factor your invoices, but that is a hole you don't want to get into if you don't have to. No sense in giving your money away. If your credit is really good, you can go in with less cash, but I wouldn't advise it.
New Truck and extended warranty, 10 cents per mile set aside for maintenance. Trade out of the truck toward the end of the warranty.
If you go in with used equipment, I would suggest $30k in cash to handle fuel expenses and unexpected upfront repairs, and set aside 20 cents per mile for maintenance. Set aside more if you can because the unknowns of taking on a used truck can be very costly in the beginning until you get the truck up to where it needs to be. You can always go with less cash if you have available credit lines, but I wouldn't do that unless there was no option.
In other words, run the company truck until you at least have the funds I mentioned before making the leap.