The real big cost would have to be unscheduled mx on any old twin. I would suggest getting your A&P because without it you will never even come close to breaking even if you want to run a commercial operation. Airplane parts are extremely expensive; mainly because they have the FAA stamp of approval (PMA or 8130). You can't just go down to the local hardware or autoparts store to get a screw (some people do anyways but its illegal). Then you have the cost of the mechanic which can run anywhere from $35/hr to over $100/hr depending on where you go. I can guarantee you that any turbocharged twin will be down for mx a lot.

A 135 operation is an expensive endeavor on its own. You will need to hire someone to help you set it up and you will need an IA to oversee your mx program. The 135 cert will require certain things be done that you don't have to do part 91. (Engine overhaul for instance isn't required for 91 but definitely is for 135.) 135's take a lot of paperwork and an in-depth understanding of the regulations. Plus, the FAA will be breathing down your neck on a regular basis.

I'm not saying its impossible to make a profit with one but don't count on it. The profit margin for any aviation related business is really small (except for the people that make the parts) but it is possible. However, one really expensive, unexpected mx cost (plan on it) could easily put you in the red.