Common advice I've received from other landlords is to avoid them because they often leak or fail and are difficult to maintain.

You should ensure the property is inspected regularly, maintain climate control to manage moisture and temperature, and address small issues like leaks or cracks early to prevent costly repairs.

In a seller finance scenario and my new posed scenario can I get both parties to agree to let me start paying 3-6 months down the road after the rehab and I start cashflowing?

Ensure your flip profits exceed the LOC costs, maintain a financial buffer, and compare it to private or hard money loans.

Ensure separate bank accounts for the LLC, and maintain proper formalities to avoid liability issues.

Our other 3 buildings (two duplexes, one 3 unit) are basically stabilized at this point so just continue to maintain them and build the reserves even larger from the cash flow they spit off.

The fact that you think the tenant is a pain has nothing to do with how you maintain the property, you could be creating legal and maintenance problems for yourself by not taking action. 

To thrive in real estate investing, you must maintain a firm grip on your finances.

Quote from @Grant Francke: For those with using Buildium for property management software, do you use Buildium's public site or do you use a website you have made and maintain?

Make sure to maintain a cash reserve (3-6 months of expenses) to handle unexpected repairs or vacancies.It’s smart to consider the “what-ifs,” but don’t let them hold you back.