Goal would be to acquire 20 rentals by the age of retirement, with some already starting to be paid off.I am in the same boat as you, and I focus on safe appreciation all day (small cash flow for safety and then focused on markets on that appreciate). 

I'm looking for ways to invest in up-and-coming markets in Southern California, such as in San Bernardino County and Riverside County.

Woah 13.85% even on an investment property is super high...I got a heloc at a 7.25% (this is variable with the market as with any HELOC) for an investment property.Talk to your local credit union.

Find one that can be custom to your needs, has flexible pricing, no long-term, high-cost memberships, and excellent customer services to determine how to best market to your leads.If you would like a referral code or more information please let me know :)

You picked a great community to join.. have you checked out local comps to gauge the market vibe yet?

Quote from @Francisco Milan: These really do not do much these days for people finding most of the listing online; however, you need to be sure that you put one out because any lead is a lead to put in your database to market to. 

The LP got it from a friend who is in some kind of real estate mentoring program.

I would choose the strategy that best fits your strengths (unfair advantages) and what the market in your area demands.

Any brokerage can be the right fit for you so long as you commit to taking initiative and putting in the work.By 'work', I mean income-producing activities (lead generation and follow-up) i.e. cold calling, door-knocking, contacting your sphere of influence, networking, hosting open houses, running online advertising, marketing (paper or online), or anything else that will get you in front of prospective clients.My first broker rarely answered my phone calls and by the time she responded, I'd often already have the answer I was looking for.

It is hard to pin down specifically, but that seems to have been a factor in a few of the ARV-Appraisals that I've seen and has impacted the refinancing process.Otherwise, it has been a great market for my clients, great cash flows and repeatability which has really helped as they have scaled.