If you are looking for a generic 80-90% of purchase to fund with a good FICO score for a normal 1-4 unit, you don't need to spend too much time on finding lenders as you have plenty of options.

IF I'm going to make extra principal payments to pay off a "low(er)" interest rate mortgage early (compared to today's rates), my gut reaction would be to pay off the highest interest rate mortgage first.

Are there any true $0 down private money lenders offering pre-approval/financing for properties in the $50-100K range?

None of the traditional mortgage lenders I've talked to so far are willing to do this, as it's more of a commercial loan. 

I was recently laid off from my job and have three issues: 1) No way to get a conventional mortgage 2) Need a place to live, and an advantage: sizeable savings on stock, bonds.I want to start investing in RE (ideally multifamilys), but for now I want to start small: buy a condo to live in, and [whenever I buy a multifamily I could move to] rent it.I would like to evaluate my high-level strategy, and get your comments:* [Soon] Get a "securities line of credit" worth around 15% of my savings.

Especially because in 10 years, if you need cash, you could consider a cash out refi, the tenants pay the mortgage, and you just got money tax free. 

To start my journey, I have been making my first steps by speaking with a realtor and lender.

Include not only the mortgage payment but also taxes, insurance, property management fees (if applicable), maintenance costs, vacancy allowance, and any planned capital improvements.

Should you know of any lender please let me know.Thank you! 

Speak with a VA lender for specifics, but you can 100% do it.Good luck!