In regards to property search, the property should most likely be close to a popular area that probably is a major attraction to the local area.

HELOC on Your Current Duplex (Great for Speed)Since you mentioned the duplex has built-up equity, a HELOC could be a powerful tool.Pros:Access equity without refinancing your existing low-rate mortgageFlexible and only pay interest when you use itAllows you to make multiple down payments over timeCons:Variable ratesRequires discipline — easy to overleverage if not carefulThis strategy is popular among investors trying to grow from 1–4 units quickly.

Student housing is our most popular real estate investment strategy here

For lenders - most notes would be held to maturity and second position liens could become more popular to bridge gaps between purchase prices and existing mortgage balances.  

Wholesaling is a common entry point for new investors.First understand, the guys on You Tube that are popular, state in their YouTube videos that they spend between $25,000 and $50,000 a MONTH to get the numbers they are talking about.

Rock Hill, SC is also becoming very popular in this area as so many people move toward the Carolinas and with the increased living costs of Charlotte.

Let’s look at Indianapolis IN; (not my market, just using it since it is a popular place)  https://www.redfin.com/city/9170/IN/Indianapolis/housing-market for a median price of $250K That's the range you want to start out in, as a beginner.Then we assume 35% of ARV (After Repair Value) to do the rehab, holding costs, sales costs and profit.

For example, your property has to be next to a popular place like a convention, concert center, beach or convenient place like a large grocery store, popular restaurant or bakery.

The implementation is not contributing to the popularity.  

Some move equity from a Bay Area property into more affordable markets (some mentioned above the Midwest is popular) using a 1031 exchange, deferring capital gains tax while trading into higher cash-flow areas.