All you need is a single place to track tenant information, payment history, maintenance, and a few other things.

How much would you suggest having in the bank after the deal goes through and I have paid the down payment and closing costs?

I currently do not have any student loans, car payments or other debts besides my mortgage. 

So my counter-offer was that I would pay them a small down payment, and have a contract saying that I must pay them in full within 18 months.

She's expecting to receive a job offer, currently unemployed but has  a good payment history.

@Jena VailThat is how we prefer to sell owner financed is the 10/10/10 rule(10% down 10% interest for 10 years)We have modified this in instances for larger or smaller down payments or changed amortization but had a ballon

2) You did not ask for a PAYMENT PLAN IN WRITING WITH SPECIFIC PAYMENT DATES & AMOUNTS AND COPIES OF PAYCHECKS!

Looking for advice on payment collection/managment software.

Also, at least in my area some handyman will do work and accept a payment plan, so maybe see if this is possible in your market. 

You should also be putting in a buyer who is very financially stable and will be competent in taking over the payments.