Any insight will be very helpful.They posted a property that seems to be a good buy, but I am nervous because I have never done hard money loans and refi after, so want to make sure I don't get tangled up as I get started in RE investing.

That activity is not for newbies, unless you are a contractor.Yes, we all start somewhere and a safer way to get started is by buying your first home, then a buy and hold SFR to gain experience rehabbing...First of all, flipping a house is not investing. 

He will pay capital gains on the difference between what you pay and his basis.

In addition to cash in cash returns and capital gains at sale, I also look at the debt coverage ratio to see if the property can support its debt service in these scenarios.  

Since you acted as your own GC, would you be able to give me more insight on investing with new constructions?

All profits and expenses are then divided 50/50, so the 50% owned by you would be subject to capital gains and the other 50% owned by the retirement account would grow on a tax deferred basis. 

Nothing to lose, everything to gain. 

Start to share your knowledge and insight with others on here and at meetings and you will attract someone who needs what your good at.

The 401k plan itself can make investments for the benefits of the 401k, you will not be able to touch any gains or incomes from those investments. 

Thank you @Aaron Cullen and @Merritt Whitman for your on the ground insights.