If this a buy/hold, you may wish to consider eliminating the inevitable issues that arise with cast iron lines and replace immediately after foundation repairs.

You state that you are having problem low-income renters so let's use the safe 50% rule, expenses are going to be higher, higher turn over, more repairs, more carpet replacements, more legal fees for 5-day notices, higher headache.  

You didn't think about management, takes, capital expenditures like a new roof, tree roots cracking the foundation, replacing HVAC, vacancy, or evictions.

We replace the lists with fresh data every 6 months and keep on rolling...But there are special groups like expired listings or tax liens or divorce that we might just hit once or twice near the event and not necessarily continue mailing to them.Also, think about how DIRECT MAIL fits into your larger marketing strategy - we also like OUTDOOR (billboards, bandits, etc.) and ONLINE (AdWords).  

@Leo B.Keep in mind that ALL of the proceeds you intend to use for the replacement property have to be held in trust by the exchange intermediary. 

Is it the purchase of the first replacement property?

Things get damaged, so there is the dealing with repairs/replacement of items.

I would like to build up to at least 25 units so i can replace my regular Joe job income of about $90k per year. 

Said he would reduce it to 50k (before he knew about the basement issue) Basement wall/footing needs replacement (quoted out at 17k) before any remodeling can occur.

The challenge is to always be replacing a new flip with one that gets sold.