Cheers...Why is it that there is always the assumption that appreciation always comes inexorably tied to negative cash flow?

I've always made the assumption that I should buy a twinhome where I could own both sides of the twinhome, but I'm asking myself to justify owning both sides over either a single side of a twinhome or a single family home. 

I am going to make an assumption.  

Hi, I'm new to BP and have been reading about house hacking on the site, using the calculators, etc.When I use the rental property calculators with the assumptions below, most properties in my area cash flow negative after servicing the mortgage.

I don’t really know that neighborhood over there – your assumption seems reasonable for students.

looking at your assumption for taxes and insurance it seems quite light.

This is a dangerous assumption.

No more assumptions for me.

Husband & Wife owned property - Husband recently passed,Wife cannot afford the house repairs (Needs new roof) and wants to sell the house fast with cash.house needs repairs but currently livable,When comparing recently sold in the area, I believe the house is priced high considering the rehab its requiringSeller wants 85,000 , 2bd1/bth 750 SQF.Similar newer and better maintained houses sold for around 90K - 100KNow heres my thoughts:Considering she's an older woman, and i am under the assumption that she wont want any new "adventures" at the current situation, given she could not afford maintaining a house she owns free an clear, I am attempting on structuring a deal where she could be my renter while paying the property (Contractor for deed)With a "Contractor for deed" i think of offering  25%+- (20,000) Down on the deal and get equitable title - Which will include the fixing of the roof using my own GC + Potentially updating house deducting the labor cost (Should she would be interested) The lady would rent from on a fixed market rent for the duration (850$)  - Paying off the 65,000 with 20 Yr monthly installments of 270$, Leaving me with 580 Gross income monthly.Summary:"Contract for deed"Asking price: 85,000Down payment for Equitable title: 20,000K20 yr Installment plan: 65,000 / 240 Months = 270$ Monthly"Fixed deal" Rent / Tenant pays : 850 - 270 = 580 Gross incomeAll in all, Lady gets to live in her own "Rehabbed house" with a long term leasing agreement and gains confidence and less hassle.I know there is alot behind these type of deals - but that's my general idea and would HIGHLY appreciate your input on this Steeeeep learning curve. 

I'm not saying to completely trust seller-produced financials but it's better than debating different modeling assumptions.