Could it be structured in a way that the seller finances, and we take care of the tank but the cost of the tank removal is credited off the loan at the end of the day? 

We understand how to structure the deal, run numbers on the deal, and most of the logistics.One thing I’m really struggling to understand is going on title for the property.

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?