It'll depend on how much you want to take out and how you want to structure your loans.

So perhaps you structure your payments at an amazing rate such as if they have already achieved 100% occupancy with quality tenants (i.e. 700+ credit scores, or 4X income levels, or 90% without pets, or other such criteria you deem highly valuable) and require cost-sharing for anything that falls below that standard - the "penalty" I was describing above. 

Ignoring the issue of trader income not being eligible, the entire $57,000 Solo 401k contribution could be structured as employer's contribution (with compensation high enough) without employee deferral component. 

You can use an entity structure to somewhat reduce the likelihood of an audit but even that can fail to prevent your file from being pulled.Generally speaking, your audit probability goes up as you make more money and file a Schedule C. 

Can either of you gentlemen - or anyone else here - advise if it would be possible to structure this purchase as a business loan?

If you delead, and maintain the standard, you will be in the clear.Here's the law (emphasis added by me):460.100: Duty of Owner(s) of Residential Premises (B) Whenever any residential premises containing dangerous levels of lead in paint, plaster or other accessible structural material undergoes a change of ownership and as a result a child younger than six years old will become or will continue to be a resident therein, the new owner shall have 90 days after becoming the owner to obtain a Letter of Full Compliance or a Letter of Interim Control, except that if a child younger than six years old who is lead poisoned resides therein, the owner shall not be eligible for interim control, unless the Director grants a waiver pursuant to 105 CMR 460.100(A)(3).

If you are all putting in money and all doing work on the property the legal structure should be pretty simple.

I'm curious to hear how those who have partnered on deals where someone else brought the funds for the deal, how did you structure the deal / returns?

Nonetheless, you numbers above would look like this:$55,000 - Purchase price, including $5K closing costs$30,000 - Down payment$25,000 - 1st Loan$100,000 - 2nd Loan - Not sure how you structure your financing, but I'm assuming this money is borrowed as well.$125,000 - Total debt on the property$200,000 - Sales price$12,000 - Commissions$188,000 - Sale Proceeds$63,000 - Check from closing ($188k, less the $125k debt)$33,000 - Profit ($63k, less your initial $30k down payment)

This could be an LLC jointly owned by the contributors or a Joint Venture agreement, but you'll be thankful for this structuring 5-15 years from now.If you are using OPM to take down a property and you'll be cashing them out, then you don't need a structure like this necessarily -- a promissory note and related instruments can be used and then once the cash-out happens they are paid off and released and you own it pure and simply.