I have begun reading David Greene's Long-Term Real Estate Investing to ensure I do this properly.What types of data should I be looking at before choosing a market for mid-term rentals?

This could mean that he is a trend setter but he does not have the deals to back that up and has not been investing long.  

I’m open to both, short or long-term, in Austin or the surrounding areas (mostly considering San Marcos).

Historically, residential US mortgage rates are near their long term averages.

Just make sure you undertand the challenges of the different Classes of properties/tenants.We operate in Detroit, which has been coming back strong for the last 5-6 years.

HELOC are great for short term cash needs, but not good for long term cash needs. 

My business partner and I have spent the last 15 years fine tuning our property management business for single-family and small multi family investors.

I've been interested in investing since about August of last year, and I finally decided on my avenue and my target markets around mid April of this year after getting over some analysis paralysis. 

If you lived in the property for 2 of the last 5 years then you do not pay capital gains tax when you sell.

Depends on your financial reserves, risk tolerance, and long-term investment strategy.