Non-payment of rent is always the path of least resistance to get someone out in a somewhat timely manner.

If you aren't going to assume the loan as discussed in #2, then you could do a "Subject To" deal where you just take over payments BUT that is really only good for the seller if they are facing foreclosure.

Is there a pre-payment penalty?

I hope everyone is well, I came here because I need expert advice from you guys, let me tell you my situation: I have a investment property that i purchased 2 years ago and is valued at 65k that does not have any mortgage and that is already paid off, I want to use that property to buy another one for down payment , the question here is: What is the best way to do it. a heloc or a cashout refinance and why. your response will be appreciated :)

Even if they stopped reporting your payments the history would be there. 

There's more to execution than merely coming up with the down payment money.

It’s true that while the 10% down payment for a second home sounds appealing, the fact that rate adjustments are now similar makes the distinction less about cost and more about strategy.A few years ago, I helped a couple in San Diego navigate this exact decision.

A question I have is, if i rent out the main unit that has 4BD-2B, is it better to get a long term renter with steady rent payments from them or rent out for airbnb?

You can also take advantage of lower down payment loan products rather than going 20-25% down on a conventional investment loan. 

An example, a small down payment initially, but the note has now seasoned for 60 months with on time payments documented by a third party servicer.