It's been a great rental bringing about $500 and there's $150k in equity.

I have one where I put down $10k and now have $300k in equity and it cash flows $500 per month.

Hi, I'm looking to buy my first RE investment property, I have 25K for down payment in a turn key SFH, but also I can refinance my house and put 150K in equity to a down payment for a MFH of 20-25 units, I would like to have the opinion of a seasoned investor because I'm new to the investing in real estate, any good advise in how to start up would be greatly appreciated.

If you have 1mm in equity could you take that same 1mm into a different investment and increase your bottom line cashflow? 

My rents to total costs (not fair market value, since I build in equity) are in the 1.08-1.17% range (monthly rent divided by total cost).

So at 75k LTV I am looking at 50k in equity, what are the odds of being able to use that equity as leverage?

The money I give a seller comes out of the $25,000 investment.When I sell to a Tenant Buyer, since I am offering Terms that they otherwise can't get, I sell the property "As Is" for a "built in equity" of an additional $10,000 to $25,000 depending on the house.

Why not rehab a property there for a better rent to sale ratio once stabilized with potentially built in equity?

That's because turn keys are generally sold at market value and don't have built in equity. 

If Tenant fails to pay rent when due and the default continues for three (3) days thereafter, Landlord may, at Landlord's option, declare the entire balance of rent payable hereunder to be immediately due and payable and may exercise any and all rights and remedies available to Landlord at law or in equity or may immediately terminate this Agreement.