I have several STR rentals in California (I co-own STRental Pros in Orange County, Los Angeles County, and Riverside County). 

Is there a risk of invalid tenants actually moving in?

I did a lot of homework on them and met with them several times before pulling the trigger. 

It will start to snowball with several rentals’ rents paying off the line more quickly.  

We are looking to use the home for vacationing several weeks out of the year and renting it out the rest of the year.

Justin,it definitely sounds too good to be true, so proceed with caution.I like to think of it this way, why would someone give you money at 3% when they can get more money in a savings account without risk? 

However as stated there is a lot more to sub 2 than just a purchase contract you have to understand all the risks and especially the risks to the people your buying the property from.. they are the ones in my mind that really need legal representation before entering into one of these transactions and deeding away a property where the their mortgage is not retired and it stays in their name and of record.

@Lenny Machin  @Beka Shea is right a soil scientist would be helpful and we have employed several over the years. 

As are several of mine in the 'good' parts of 77004 (i.e. west of 288). 

Several years ago, you could find a rough house in 35810, 35811, 35816, or 35805 for less than 100 a square foot, rehab it, and rent it for cash flow.