You listed two but in broader terms financing (assumable, owner financed, sub to, fha, new 95% OO LTV that in most ways is superior to FHA, dscr, F/f conventional, NACA, VA, etc), value adds (development including ADU, upzoning, traditional rehab, sophisticated value adds that leverage localized rules, coop/tic, etc),  property types (residential, commercial residential, land, storage, MHP, industrial, office, camping, hotel/motel, etc), passive/active investment methods (GP, LP, REIT, nnn, self managed, pm, flipping, mineral right, lumber rights, etc), residential rent models (LTR, STR, MTR, rent by room, student housing, arbitrage, etc).  

With these methods, you would be able to reach a large number of sellers without necessarily spending too much per contact, as compared to direct mail.

We have purposefully stayed away from Beloit... there are some decent properties there, but we found that there are not enough quality tenants to occupy the homes.

Other methods? 

Cold calling is only one marketing method.  

For example, if a 1,600 sq ft main house is appraised at $100 sq/ft, what could I expect a 600 sq ft ADU of similar build quality to be appraised for?  

The investments will vary drastically in price based on the neighborhood your looking in & will translate to the quality of tenant. 

@D Kirk, I am all for saving money when you can get comparable quality work done for cheaper.  

But the question is whether one can actually achieve the proforma cash flow on older properties with a less economically stable tenant base.Based on your picture, you appears fairly young.My advice is buy properties in quality locations think long term 10 to 15 years - slowly and safely build your portfolio - equity and cash flow will increase over time and in 10 to 15 years you may become financially free.People talk cash flow is king but one can NOT generate financial freedom cash flow with $250,000 or $500,000 invested (equity).Even at a 10% cash on cash return, one needs $1.2M invested to generate $120K a year $10K a month.You want property to be self supporting so it needs to cash flow but once that is reached the focus in my opinion should equity growth which increases net worth.

Seriously, they are a lower-quality renter that poses a higher risk.