Your in over your head with that many properties, I have been to numerous Philly auctions and there is a sea of people who will just outbid you to the point it’s not even a deal. 30 properties is not feasible and chances are half of them won’t even be auctioned that day.

There are numerous part time agents in this area doing in excess of $3 million annual volume.I know this comes down to my own goals and risk tolerance, but do any of you have an opinion or advice when deciding who to go with?

That property has been for sale for a while on numerous sites. 

While they're great and have invited me over numerous times for a beer and BBQ, I don't want to blur the lines of our Tenant/Landlord relationship.

Along the way the GC and subcontracted plumber mentioned numerous times that they found great news or thought of more efficient solutions (i.e. the mainline plumbing under the basement was intact--that they had assumed was all broken and needed replacing).

They could be unsavory for numerous reasons (as an extreme they could be child molesters or drug dealers).  

I imagine there are multiple suppliers of such reports... what ones do you like the best?

Yes too numerous to describe in a short reply - plus ethical issues

mid-sized apartment building, you should be working with an experienced broker and/or attorney who should be able to advise you on what materials and {diligence} conditions should be identified in your APS.Building on what has been stated above, be sure to include the collection of estoppels from all tenants (and suppliers) and to have the clock on your diligence period start only once the Vendor has supplied the information requested of them (in the APS).Other items which we typically request as materials include: all leases and contracts; blueprints, engineering diagrams and assessments; existing environmental assessments; authorisation to pull the permit and work order / citation history on the property; finances for the past 3 years; current rent-roll along with economic vacancy (physical vacancy, concessions, evictions); list of recent CAPEx; statement of reserves (note: this should be included in the financials, but you never know).

Then he discovered the judge from Housing court had previously put a demo order on the house due to a past owners negligence and not correcting numerous code violations, also recorded on title.