Biggest flow I see is renting to family and Friends..

Rent to Value ratio, this is the biggest determinant of cash flow!

The biggest challenge has been carrying out these processes while working a fulltime job and staying committed to the process it will take to develop a life-changing portfolio.

So you lose some of the biggest leverage advantages of RE investing. 

You may find lumber prices are 2x more than last year but there are alternatives.Chris Clutch IndustriesHe's right my uncle is one of the biggest construction suppliers of stick and other products on the east coast. 

What are the biggest challenges and hurdles you faced while flipping those properties?

Income etc....Besides those my biggest thing I avoid in a tenant is any sign of aggression or combativeness.

My biggest issue with many of these operators is signing contracts with no intention of closing, or not finding a REAL buyer and then using BS escape clauses to get out of the contract.

The biggest cost from an eviction will likely be loss of rents and turn costs. 

(Which is the biggest barrier to entry)Setting an agreement ( x properties per month for y months at z price) land contract with financing might be the best option.