Hoping someone can help me give advice to my friend.She is looking to buy her first house in Wisconsin. It is an old farmhouse in central Wisconsin, currently occupied. As it is in a rural area, it has a septic fiel...

This prevents the buyer from seeing the assignment fee. 

Not having all this discussed and agreed upon up front is how many partnerships end up in disaster --- although, having it does not prevent it... 

Utilizing experts’ knowledge can prevent you from buying the wrong investment property, one with maintenance, location and even legal issues.Read this article for detailed information https://www.forbes.com/sites/f...All the best!

They would charge less than $400 a month, have a solid lease, know the law, know the market, can deal with difficult situations quickly to prevent them from growing, etc.

Mail the air filters to the Tenants and make them change them.Your current method is OK for a few rentals, but it's not going to scale and I have to question the cost:benefit of it all.A traditional property management company doesn't have an incentive to perform preventative maintenance or to keep tenants happy, while the self-managed property owner has both. 

How to prevent this kind of potential damage/trouble ?  

It's part of the game, always better to be preventative!

Can I still file for an eviction to help prevent this from happening again? 

Still, there are disadvantages that may prevent a buyer or seller from signing on for owner financing.Advantages for BuyersCan provide access to financing that a borrower may not otherwise have qualified forEnables buyers to finance homes that don’t qualify for conventional financingLets buyers and sellers shorten the due diligence period for quicker closingReduces the cost of closing by eliminating appraisal costs, bank fees and—if the buyer so chooses—inspection costsEliminates down payment minimums imposed for government-backed mortgagesAdvantages for SellersAllows owners to sell their property as-is, without having to meet a lender’s appraisal requirementsPresents an investment opportunity with better returns than most traditional investmentsShortens the selling process by reducing due diligence requirements and eliminating the lending processStill offers the ability to sell the promissory note to an investor for an up-front paymentLets sellers retain title to their home—as well as money paid toward the mortgage—if the buyer defaultsDisadvantages for BuyersOften involves higher interest rates than a traditional mortgageMay require borrowers to make a balloon payment at the end of the loan termDepending on the borrower’s creditworthiness, the seller may not be willing to provide owner financingSeller’s mortgage may include a due-on-sale clause that requires them to pay off the mortgage upon selling the house, thus precluding them from offering owner financingDisadvantages for SellersExposes sellers to the risk of non-payment, subsequent default and—in some cases—a need to initiate the foreclosure processPuts seller on the hook for repairs and other consequences of deferred maintenance if the borrower defaultsFederal law may preclude sellers from offering owner financing, limit balloon payments and require the parties to involve a mortgage loan originator.All the best!