Looking for a recommendation for a plumber that does sewer cleaning/camera inspections.

Also, for future issues like this, can you purchase a camera/dvr system and point it at the drop box?

Yes, at this point in the cycle, sellers want big deposits, because they can get them, and because they want the buyers over a barrel in case the market turns against the seller before the deal closes.Obviously, putting up a bigger deposit, especially if it goes hard early (or immediately in some cases), raises your risk substantially.You definitely want to get a contractor or engineer in there, and absolutely someone with experience dealing with properties of this size.With a property that age and in that condition, you should also look into hiring someone to send fibre optic cameras down the water and sewer pipes.  

(Obviously I will continue my due diligence after the walkthrough.)I plan to bring a notepad with plenty of paper, a strong flashlight, a good camera, and all the insight offered here.  

Occupancy is currently about 70% (one tenant, a photo/camera store, closed up shop because of the switch to digital photography). 

The prior owner is currently installing additional lighting and security cameras.

When selling a camera on e-bay for $50, a 1% fee isn't noticeable.  

Plumbing Lines and Septic Inspection - For $250 approx. you can have it inspected with a camera through the clean out.3.

There is a coded gate for access to the facility and it does appear to have security cameras though I have not visited it yet.

Or install security cameras and report them to the police.