Tell the current tenants to post a notice saying any packages NOT addressed to the current occupants (list the names) is refused and should not be left. 

Not 55+ exclusively, but a 14-unit lodging house (aka: rooming house/SRO/single-room-occupancy/co-housing).

A SFH generally has more liquidity which is to say more options for turning back into cash, not just in fact that it can be a conventional market sale to a owner occupant but also the more finding potentials for a possible buyer.

If one comes across a property that is a completely renovated multifamily commercial property but has no tenants, should one focus on using the actual potential income of the property (meaning total income the property could have generated assuming 100% occupancy rate over 12 months) since the actual income doesn't exist to calculate the NOI and purchase price?

For those looking to get into the industrial space, things to consider:-Zoning & Occupancy.  

I look for fixers. low occupancies, value add situations, good zoning all in areas that are likely to appreciate.I can get a good deal, it is in area of nice properties and has other characteristics that will add value over time.

The former owner/occupant of the house I bought paid a grossly inflated price (even for that time) for the house in 2007, and his only contribution to the house was to paint each room a different garish color.

They tax at a 6% ratio as opposed to a 4% Owner/occupant..which usually figures up to three times the 4% and then a couple hundred on top of that.

New inventory, (residential housing) would not be released, even it was fully handed over by the developer, until the banks and property management companies who controlled the new inventory were satisfied that other surrounding buildings had reached a healthy occupancy rate.

You can also look on craigslist (just make sure to vet them carefully and ask for references because many are cheap) or Angie's list (just know that most of them focus on owner occupants and are too expensive).