Help appreciated with seller finance deal

[Alec Anderson]

2 duplexes for 162k

Zillow estimates they are worth 84k and 85k each

They are both mostly brick, identical, and right next door to each other. All units 2br 1ba.

One roof has a leak, so it needs repair or possible replacement.

The street is probably C or D class in my opinion, mostly similar quality duplexes. There's some nice neighborhoods (300k+ houses) a block away, and some sketchier seeming neighborhoods a block away as well. 

All 4 doors rent for $600 each, so $2400 a month. 2 tenants have been there for multiple years, 2 less than a year.

Seller wants 5k down each, and $550 a month each. So $1100 per month, with $100 going towards the principal, for 5 years, then a balloon payment.

Taxes for the 2 combined would be about $275 per month, and I'm guessing worst case $200 for insurance.

So 2400 income and 1575 in expenses, and lets say $300 per month for repairs and other unexpected things would leave me with an estimated cashflow of $525. 

How do I figure out the actual interest rate I'd be paying the seller? Does this seem like a reasonable deal? I believe the seller has his own financing on the properties, so how would I protect myself from him not making his payments, keeping all my payments and getting foreclosed on?

The 10k down with no banks is really nice, but after 5 years i'd have payed him 66k in payments, plus the 10k down, and only 16k of that would go towards the actual 162k principal. I'm too new to know if that is normal or not.