Hey First Timer,
My advice is that you can NEVER have too many inspections, especially if your gut is telling you to conduct a specific one. As an agent and broker, I will always discuss with my clients all potential inspections a home might require before we even commence the actual house hunt. I also provide rough estimates as to what to expect cost wise. From there, I let the buyer decide which ones they want to add on (in addition to the ones I require ALL my clients to conduct for everyone's protection: general, mold, chimney, termite, and pool if applicable).
A general home inspector typically does not provide a thorough sewer line inspection. To do this, they need to put a camera up the line to evaluate blockages, etc. You are then provided with a disc to watch of the lovely contents within your sewer (it's great to watch over a crackling fire and glass of wine as well as your Chimney Inspection video) as well as a report and recommendations. I sold a house in the hills north of Sunset on lower Kings. I would have naturally thought the line was connected to the city...WRONG. We did a sewer line inspection which revealed it was an a septic tank which needed to be completely pumped (septic was unknown or undisclosed by seller) and that the city had made a line available for connection in the last few years if he wanted to connect down the line (pun intended). That $1500 inspection yielded a $3500 credit in return, a pumped septic paid by seller, and knowledge we didn't know prior.
One final thought about your situation. By doing your complete due diligence, you not only alleviate liability for your agent, her brokerage, and the seller but you are also getting peace of mind. Any agent who is reluctant or "acting annoyed" because you want to pursue this further is suspect in my book. Some agents want to scoot by with as minimal inspections as possible b/c the less problems discovered, the less likely you are to want to back out. Potential deal breaker = loss of commission for agent.
Best of luck to you. When it comes to any big decision in life, always go with your gut and know that we as agents are here to look out for your best interests at all times, regardless of financial return.
All the best,