The point is to have the tenant help or fully pay the mortgage.This is where the question comes in.

Issue with the line that you will run into is you will be paying for unused credit fees.

The str will close before the land sells and then I would like to recast the loan to pay off a portion with the land proceeds.  

You might end up paying taxes on the personal side for money you wouldn't even distribute to yourself. 

Tenant is paying $1300 a month; market rents are $1800.

I have 3 condos in San Diego that I'd like to refinance and take equity out of (to pay down debt).

Then use a multifamily FHA loan to pay off the loan I used to build the homes to hopefully lock in a better rate.

If I want the money out for a longer period of time, longer than a year, I prefer to refinance.A HELOC is much like a credit card, in that you pay what for the amount you use, and it can be repaid and then drawn on again. 

Or would you have to pay commercial property taxes since it is technically considered an investment?

You have to be 18, but you can typically find someone willing to hire you, pay for your license, and train you.