Many people choose NOT to have this, because the administrator will then vet every investment you make to ensure you do not invalidate your IRA.Invalidating your IRA means that you've done a prohibited transaction and the entire account is then deemed as distributed to you.If you don't want checkbook control, then I recommend a plan administrator such as Equity Trust - www.trustetc.comIf you are sure that you're aware of all the rules around prohibited transactions and disqualified people (who you can and can't do business with), then a plan administrator like Sense Financial would be a good place to start talking.  

We cannot breach our guests' trust by playing games with them. 

I currently do not have enough funds to fund a down payment and am trying to find the most efficient way to save.

if you never miss a payment and the bank doesn’t call the loan due.

But how can we work together if neither of us trusts the other?

There are some great suggestions here on down payment, interest rate, servicing, and underwriting the borrower.  

When it comes to managing your mortgage, most homeowners are familiar with refinancing as a way to lower monthly payments or take advantage of better interest rates.

(Even went beyond the second page of Google.)I have an investor who wants to partner on a deal by providing a loan for the down payment.

Here are some general guidelines: Lien Position1st Position LienEquityMinimum Down Payment of 20-25% (a 30% equity to sale price is preferred)Down Payment made in certified funds and not borrowedMaximum Funding of 70% ITV (Investment to Value)SeasoningMinimum of two monthly payments already made12 months or more preferredPayer CreditCredit Score of 650 or higherNo major derogatory trade lines (No 90-180+ days delinquent, foreclosure, repossession, bankruptcy, etc.)No decline in credit since purchase of businessTermsPrefer 60 months or lessMaximum of 72 monthsNo Balloon Payment preferredCash FlowBusiness has positive cash flow1.25 debt service coverage ratio preferred.DocumentationPersonal guarantee from a creditworthy individualStandard documents including Note, Security Agreement, and UCC-1 filing.Tax Returns on the business that substantiate profit and loss statements.OtherPayment history current and verifiablePayer Interview indicating buyer is satisfied with business and comfortable making paymentsMotivation for selling the note is not a result of a deteriorating businessConsideration of high risk businesses or geographic areasIn these situations, sometimes a partial is a better option. 

You have trusted contractors that are charging a fair price ?