Do I list in the Rental Increase letter "All terms from the previous rental agreement will remain in effect" if I have never seen such lease or terms?

Here are more $$ details:Buy and Hold:Yonkers 4 unit multi-family house (5 minutes to Metro North station)Monthly expenses: $5kCurrent rent roll: $6k (can be increased with more renovated units, and current landlord keeps rents low to keep tenants)Renovate and Flip:Brooklyn (Bed Stuy/Bushwick) Condo 2 Bed 1 Bath 850 SQFT- 5 minutes to J/M train station$500k Purchase priceMonthly expenses: $3.2kEstimate monthly rent: $2.2k, $2.5k renovatedThank you all for your time and input!

Here are more $$ details:Buy and Hold:Yonkers 4 unit multi-family house (5 minutes to Metro North station)Monthly expenses: $5kCurrent rent roll: $6k (can be increased with more renovated units, and current landlord keeps rents low to keep tenants)Renovate and Flip:Brooklyn (Bed Stuy/Bushwick) Condo 2 Bed 1 Bath 850 SQFT- 5 minutes to J/M train station$500k Purchase priceMonthly expenses: $3.2kEstimate monthly rent: $2.2k, $2.5k renovatedThank you all for your time and input!

The housing development of condo type in my city has been increasing in the last few years.

If they are marketing and bringing you new properties, then a percentage or a flat finder fee for each property may be appropriate.If their role is purely administrative and they are simply managing the rentals once they are received, then I would only consider a periodic bonus based on the increased workload.

Over the Last 2 years I have become increasingly interested in the opportunities that real estate can provide for me to get out of the corporate rat race and doing something more freeing. 

You might think that without money changing hands that it can't be binding as there's no "consideration" - but my attorney tells me that courts have held that the offer itself is a "thing of value", so the contract itself can be the consideration.It's unlikely that the unsophisticated FSBO will end up bludgeoning you with this, but if the brother-in-law turns out to be an attorney, it could go sideways.Strategically, I'd get face to face with him. 

Putting a face to the name will instantly increase your credibility to them.

A few considerations which I am sure others would advise you:1.

In the meantime you will have created great cash flow that increases your monthly income!