Even if they just have them on as an advisory role, having them on the LLC helps you get a better deal on funding and their experience will help you to get the project completed more quickly. 

HELOCs only make sense as short term funding - especially with rates where they are now. 

To address this, we agreed that I would work in the business for six months to verify the financials, which so far align with his POS system.My main issue is that at 23, I don't have strong financials to get a conventional loan at 75% LTV with a ~7% rate, and the seller's lack of financials complicates bank approval.

My goal is accumulate properties while I’m still living at home, to eventually be able to fund building my own house with the cash flow from the properties.

Yes, in my opinion because of the terms and rates of private money it is best used for short term acquisition and rehab.

We will also birddog the best opportunities on the contracting side so i figure we can boost out hourly rate up to at least $100/hr (from 80), and jump from the build to contract jobs to take advantage (we have our own heavy equipment and dump truck).

I imagine this would take close to a year if all went well, and eat up a lot of funds.

I am looking to try and build up funds to start purchasing properties.

This is almost aways permissible as a source of down payment funds.

My thoughts:  The HELOC might be a good emergency fund, but basically it is a large credit card.