Jack,IRA LLC is not a DIY project, you should use a professional who can set this up correctly.

For example, if the proposed project estimation total is 90K, can a group of investors invest 30K each, each gaining their fixed interest on it based on their contribution?

Scott’s book The Book on Flipping Houses, I am finding that this strategy is very similar to being a project manager and managing the triple constraints (cost, schedule,performance), which I enjoy. 

I have a HELOC I use, and two lines of credit, but I use them for short term projects.

Peter if you go new subdivision pay attention to the size and solvency of the builder.The small builders that do one off homes or small to mid size cluster subdivisions those are the ones that usually can be in big trouble if an economic shift happens.Look at new build permits coming online for the area and the price range you are looking at.The big builders tend to be more experienced and can still finish out  projects with down turn so you do not get left with a pipe farm development where some homes and some empty dirt parcels.Nothing is foolproof but this can help.

You can invest in others and their projects as a private lender.

As an example, will it need siding in 5-7 years, this could be a $20k project.

I've forgotten the reason my lawyers had advised against it but usually the people can't qualify for a loan and while I own my properties free and clear, carrying the mortgage doesn't achieve my goal of generating cash so I can reinvest or work on my other projects. 

I travel with work, building project up and down the east coast.

That will define your price and rent point, which pretty much answers your ZIP code question.As an out of state investor you don't want to buy projects.