Are you talking single-pilot or single pilot-in-command?
One you only need a letter of compliance for, the other you will need all of the mgt positions, manuals, MELs, etc. as appropriate.
What aircraft and mission did you have in mind?
The FSDO may ask you:
Have you researched a market? (I'm told the FAA looks bad when "airlines" -you're considered an airline if 135- go out of business, so they want to make sure you've got a business plan in place)
Do you have permission from the airport/port authority? (They may not want another charter/airline at their airport and certainly can prohibit you from operating there. Even publicly funded airports can. At our airport, the port authority gave us hell about starting a flight school...a flight school! YMMV)
Why do you want to start a 135 company? (see above comment about making a small fortune)
In the certification process (that we just re-started), they'll go over how you can be safe, etc. It's a pain, but a good learning experience.
The biggest expense if you already own the aircraft will be insurance and maintenance. I can almost guarantee the Airworthiness Inspector will find reasons to deny your aircraft if not ground it.
If you've got a market, enough cash to survive on for 6 months (minimum) and a support system (family, staff, whatever) then go for it. You'll learn a LOT in the process.
-mini
PS
Don't dawdle. When they want something...get it to them. If they ask you to jump, ask "how high?". The reason we had to re-start was my biz partner got cocky with them and "magically" they never recieved any of our manuals. Concequently, we're going through a different FSDO. NOT fun.
Good luck!