The fact that you have already called them a client can put you at risk of not diligently protecting their interests.You would probably be best off to get them to list the property with you and to sell it for them.

I assume there should be some formal agreement to protect me and ensure I get paid.

*IF* you get serious on this approach, protect yourself and have an attorney draw up these papers in keeping with your State requirements.

One thing my CPA brought up was possibly looking into an LLC strictly for protection of my other assets.

I'm told to put my investment into a Trust fund so that I'm protected.  

Following are the similarities and differences between the solo 401k and the self-directed IRA.The Self-Directed IRA and Solo 401k SimilaritiesBoth were created by congress for individuals to save for retirement;Both may be invested in alternative investments such as real estate, precious metals tax liens, promissory notes, private company shares, and stocks and mutual funds, to name a few;Both allow for Roth contributions;Both are subject to prohibited transaction rules;Both are subject to federal taxes at time of distribution;Both allow for checkbook control for placing alternative investments;Both may be invested in annuities;Both are protected from creditors;Both allow for nondeductible contributions;Both are prohibited from investing in assets listed under I.R.C. 408(m); andNeither may be invested in your own business. 

It seemed like a good idea at the time to have all our future assets protected and a corporate veil that came with encorporating with their company in another state.

And, do I need a Pour Over Trust on top of that holding company to protect myself further?

You just need to make sure your contract includes provisions to protect the deal from your perspective.A few things to consider.  

You need the insurance so as to create the Loss Payee clause protecting the HELOC.