My goal is to purchase 15 units in the next 5 years, and produce a cash flow of $20k per month within the next 10 years.  

Gain is your net sales price minus your adjusted cost basis (purchase price plus capital improvements minus depreciation). 

Purchase price: $675,000 Cash invested: $250,000 Country estate with 2 houses.

Financing depends on your scenario and what type of property you're purchasing.

The biggest determining factors for STRs in the area are purchase or build price (and associated mortgage) vs rental expectations, that will help you determine net cash flow.

@Mike ReynoldsSeller financing is allowable for Solo 401k purchases.

Purchase price: $110,000 Cash invested: $30,000 Hobby Farm On 10 acres2300 sq ft house with 8 out buildings - Large Barn, Animal/hog barn, Mid-size machine shed (2) 25' x 35' / 30' x 50', Large Machine shed 60' x 60', Grainery barn, Entertainment Barn, 3 car garage What made you interested in investing in this type of deal?

These estimated costs should then be reconciled back to actual costs or the purchase price.Have you had a cost segregation study performed?

What's the purchase price? 

Self-proclaimed Section 8 guru's online claim to do the latter and then find Property Managers after they purchase the property in whatever state, but I don't know how real that is.