I have a pretty unique situation.I have a family member who wants to help me with my real estate pursuit/dreams to financial freedom.

Ask what the range of interest is on the type of loan you'll be requesting or the current rate as applicable.

Most properties that I'm looking at are in the 35-75k purchase range and ARV values of around 110-150, with average Reno cost of around 20-30k with most projects taking 90 days also since there are a lot of FHA loans on these properties it really doesn't benefit to flip it any faster. 

I think it would be reasonable to expect your total investment to be up in the $105-110k range on this deal.  

We've been looking for other houses but haven't found anything comparable in the same price range and location.

But, if you dont have a homogenous portfolio, ie a 1 million dollar commercial building vs 3-50k SFRs, you may want to have stronger protection or separate LLCs because of the unique exposure differences as well as asset loss potentials.  

The new buyer then pays you a one time assignment fee (most of mine range between $3k-10k).Some folks like doing sandwich lease options (this is where you yourself are under a lease option agreement with the seller, and within the lease component it gives you the right to sublease to someone else).

I have looked at the "city-data" website for Yonkers, NY and I know that the population increased 9% since 2000 and the median income is in the $50,000 range. 

Specifically in the price range above 300K.

If you are in a unique situation, the answer may vary.