First, if it’s the company that’s going to be doing 135 work, they would be doing operations under their certificate. You wouldn’t need one. Second, you owning a plane for them to use will only work if they’re interested in leasing it from you.

As to starting your own 135, the difficulty and costs vary greatly depending on the FSDO you’re dealing with. There’s still tons of paperwork. I worked for a school that tried to add a single pilot 135. Writing the opspec took several weeks of work, and that was with one from another company to copy. Then, after sending in all the paperwork, the owner gave up after more than 2 years without the FSDO having even looked at the application yet, and basically being told it would be several more months before they could even give him a timeline on it. That said, I’ve known people in other places that have managed to get one in as little as 6 months.

The preferred method is to buy a company that has gone out of business in order to get their certificate. You then “move” the company to wherever you are, rename it, update all the paperwork, and you’re done. Catch is, this can easily cost tens of thousands of dollars.

Unless you have a bunch of money, and a strong desire to start a business that has a low chance of becoming successful in the long term, it’s better to work for someone else.