Point is it's actually more than you think when you add it all up.That said 5% flat would be a very tough sell for only 20 units. 

I am trying to decide between two property management fee structures offered by a company.Rent: $1340Option A:Monthly Management Fee: 6%New Tenant Leasing Fee: 70% of RentLease Renewal Fee: 35% of RentOption B:Monthly Management Fee: 8%New Tenant Leasing Fee: 70% of RentLease Renewal Fee: $150 flat feeAll other terms remain the same and both the options offered by same company.

If you can repay the loan, in full, within 12 months, it costs a flat ~$4,000.Best of luck, whatever you do choose.

Assuming you've correctly estimated the rehab budget, ARV, and yearly expenses, once you've rehabbed the property, lived in it for a couple years, and rented it out with positive cash flow (see 50% Rule), assuming the housing market stays flat or continues to appreciate, you should be able to take out a primary mortgage on the property to pay off your parents HELOC.

In your opinion, what would recommend a flat water/sewage amount be included in the rent or a fluctuating amount based on usage?

I can't buy my property any place else for anywhere under $650 but those are in different neighborhoods.Several years ago I bought a 2 flat apartment building at 2500sqft for $220k.

@Thomas Ackley's answer was perfect, LOL - a lot of these risks can be mitigated by choosing a really good market that has favorable legislation and local economics/resources baked in.  

I do have 10% baked in to my numbers to cover PM. 

When you sell, you would owe taxes on whatever the sale price will be minus $50k, so your refi is baked into this total gain.PS. an unsolicited piece of advice: "super DIY guy" mentality is super dangerous.

So, i would rather it be a flat fee.