I'd slow down and focus on quality over quantity. 

Another regular mail house.Figure out the quantity of mail you plan to send.

@Matthew RembishWhile I do agree with @Charlie Brown that such a small quantity of letters could be done in house and thus save you money, I also understand the value of your time. 

They are cheap but can be a pain if needed in quantity fast.  

In this market it costs around $200/yr for a twice-a-year check up (furnace in fall, ac in spring).I guess I'm leaning toward putting out requests for quotes for the whole portfolio to see if I can get a quantity discount on the checkups. 

One of the questions you might ask is whether there is a quantity discount, some are not readily advertised but may be available.

At first, I thought of developing my own spreadsheet that includes the products I need for each of the units, the purchase date, the quantity, etc.

Have also visited Lake Harmony and love it there- plus it is closer to Philly. 

The only things that limit your success are the quality and quantity of your network.

Here marketplace provides significantly more quantity but quality comes from Zillow.