This, in turn, allows for higher rental rates or quicker sales, directly impacting the ROI.I could remember when an investor purchased an old, three-story commercial building at  a bustling downtown area.

Head over to bank rate or and take a look at the mortgage calculators, plug in the numbers and get an idea of what the property needs to generate to satisfy the intended debt.You still have to underwrite the deal for the non-finance expenses as well(typically 40-50% of the gross rents). 

Land is very risky, and being in hawaii makes it even more challenging - thus reason we see higher rates on land. could it be less, yes, it could be in the 15% range but i would be shocked if someone would be in the 10-13% range.

Is it possible to get a FHA at a good interest rate with just a good credit score and proof of my start date?

If you pay cash for something and are able to refinance at a rate that makes sense to pay off your HELOC, that's the only thing I could see making some sense. 

I know a lot of investors have done very well in Peirce County but I just haven't learned or know enough about that market to make an investment there.Washington state properties are expensive and can be difficult to make breakeven let alone cash flow at this moment due to the combination of high prices and interest rates. 

If you're $400 below market rate I'd give them chance to "catch up".

Assuming I also use the Airstream for lodging while traveling for personal reasons, would I just take a pro-rated amount of depreciation for the amount of use that is business vs. personal?

Being owner occupied means getting the lowest interest rate, and a conventional or fha loan. 

But at a lower rate compared to other cities that are growing.