The owner gets to defer taxes on the second half of his capital gains. 

I was forced to cash out my 401k in payments to minimize tax penalties, using it to complete house projects and pay off some debt.

Its tax time so 1099/w2 or bank statement.   

We have substantial closing costs on the sale, primarily transfer tax and realtors' commissions.

Look at property tax sales.  

You have the capital gain taxes on the backend.

Property taxes ought to also be considered.

Discuss any pros and cons with your attorney and CPA to weigh the advantages and disadvantages, particularly regarding liability protection and tax benefits.

So no tax returns, DTI, etc.

One thing to note, is that if you are doing flips in either Solo or SDIRA the IRS is likely to see that as running a business and you could be subject to UBIT (unrelated business tax) which scales up to 37% so it can be pretty hefty depending on the numbers.