As long as there is equity above the first and the second is protected.I would have a qualified real estate broker gives me a broker price opinion or BPO and then subtract the first position mortgage unpaid principal balance.

Subtract 12 to 16% economic loss depending on the submarket and property class.Find out expenses for the similar properties and assume yours would be about the same. 

After subtracting all the expenses, there will only be about $60 cash flow each month.

Now subtract 10% from that for your Property Mgmt fee and that'll bring you down to $37,800.

Take the initial $70K, subtract $5K in repairs, and $5K for your profit, then $60K is the most you can pay for the home.  

I subtract annual taxes, annual insurance, expected annual maintenance cost, and an annual reserve to give me cover all repairs that will be needed over 10 years, expected annual inspection permits, and the 10% property management fee.

Then from the beginning of the second year they will try to keep the property rented and and if some problems occur they will subtract some money from the rent to the maintenance.  

Subtracting the $200/month from the trailer park, you are now pocketing $200/month beyond the $1000 you had to spend.

I understand that you're supposed to take 70% of the ARV and subtract out rehab costs.  

I am also interested in knowing the NPV of the total amount I will be paid through all 18 years and subtract the purchase price to know what are my earnings; what interest rate do you use: current loan rate / gross yield / finance rates , etc.?