If your goal is cash flow, then your best bet may be to live in it for 2 years, and then take that tax free gain (250k single person, 500k married) and buy elsewhere. 

I can bet if inflation runs then Boston prices skyrocket, but I doubt Austin does.

If someone is willing to pay you $500k / year, I'd bet that you're a pretty high value / hard working person.

You can just answer it without the jazz, and I bet you know a few places right off top of head where people WANT to live, right. 

I bet that you can pay off this debt in 1 year or much less.

I would say for most people, the 1st position heloc is no a good bet, especially if you have a 1st mortgage rate of less than 5%.Using this strategy with a 2nd position heloc can be just as powerful, so you can have your cake and eat it too!

With cast iron it's safer and can last longer.

Your best bet is to get on the phone, get multiple opinions, multiple quotes, and make decisions on the type of build before pulling the trigger.

People seem to buy mainly for appreciation, but I'm not sold appreciation will continue to rise the way it has the past 5 - 6 years, so feel like CF is the safer investing avenue for me. 

(I know a lot of investors who are doing this, but I've been surprised to find out that it's a new concept that others haven't considered.)The more units, the better in this scenario because you can hedge your bets with more units.