I had access to the cash through 401k loans, Roth IRAs, credit card lines of credit and some money in savings to get me through.

But I did read and hear that traditional lenders don't like LLCs.

Whether it's traditional rentals, MTRs, or STRs.

Our CPA is encouraging us to contribute max to our Traditional 401k, however I am wondering because this is a pre-tax deduction will this affect our "buying power"/mortgage amount in the future?

On traditional flips, you will normally pay ordinary income tax on any gain upon the sale since this is considered "active income" which means you pay whatever your individual income tax rate is on the profit.When it comes to flipping, there is so much involved from the tax side that you really need to talk to a good CPA that specialized in real estate and pay whatever they want you to to help you time things correctly and structure the deals so that you win. 

I'd keep rent low, no car note, frivolous spending minimized, retirement accounts maxed(401k & IRA), savings a high allocation.

I.e. dissolve the LOI and attempt to purchase through traditional method once it hits the housing market.

The only thing that I have seen post IRA is 0% financing options.  

Is it a land contract or traditional mortgage and note.That’s what title company will ask so they can create the documentsIf you are the buyer then you absolutely do not want a land contract and want property conveyed to you with title insurance

If your currently builder doesn’t offer any internal services to transition you, you could: - Borrow agains you IRA (401k), depending on the cost of the deposit, and pay it back when your old home sales and it's off the market.- Use a HELOC (80% - 85% of appraised values of current home) to use as deposit for the construction loan requirements (10% - 30%) and the debt would be concluded during the sale of the home.- Gift from family member as a deposit, and payback when the home salesThe question is really broad, so if I have misunderstood you question please let me know.