I'm primarily focusing now on investing instead of the traditional sales.

Specify reasonable items that you'll fix as a landlord and what maybe unreasonable.

All I am buying a house with the strategy to resell it to the current tenants... Seller needs immediate money, so I am buying it cash for 100... Retail value 140.

I understand that tenant could not buy it right away ...

But in my case, since this is a rental, the ARV does not matter (until I sell).as far as knowing what things cost... it's kind of like groceries - you have no idea what a 5lb of gluten free flour costs until you start buying it regularly. rehabs are almost the same way, just not as consistent as grocery items.

mother-in-law, mother-daughter, jack-and-jill, 'Summer Kitchen' and/or maid's quarters yada yada  - as many mansions and other estates traditionally do -  if its a SFD that means per code/ordinances etc u cant rent separate tent dwelling/apartments within it - only maybe a room or so.thats why instead, large SFDs with a duplex layout (or maybe even a 'back house') to spare will 'rent out a room' to avoid grossly breaking zoning and C of O rules for a single fam dwelling. of course that means either the kitchen is inherently shared or one of the persons (ie, a college student, or a raw vegan) simply does not need access to a traditional kitchen whatsoever.

Individual #1: Taking out traditional mortgage with 10% down to purchase propertyIndividual #2: Managing restoration and sale, taking share of profitsThey need money for the restoration budget, estimated at $40K.

Are you looking at traditional bank financing?  

I currently have some money in an old 401K and a traditional IRA that I would like to roll over into a SDIRA.

You will have vacancy, you will have bad tenants, you will replace expensive items. 

What is of importance is their 30,60,90 day pay records, credit limits, percentage of credit used, what type of credit items they are defaulting on etc.