Deal or No Deal

[Bob Yon]

I've got a deal that I think is a good one, but am a little uncertain about some of the numbers:

Here is the deal:

Duplex with $750/mo rent
Purchase Price $28000
Down Pmt $18000
Owner finance $10000 at 0% for 5 years
Closing Costs $2000

I normally use a 40% exp ratio which includes taxes, insurance, vacancies, prop mgmt, repairs and reserves. 
Property needs about $3k of clean up and repair before tenant moves in 2nd unit (1st unit rented)
Also will need a new roof ($5k) within a year or so

I've calculated NOI as: Gross rent - Expenses = NOI 9000-3600=5400

I've calculated Before Tax Cash Flow as NOI - Debt Svc 5400*2000 = 3400

I calculated the cap rate as NOI / Purchase Price 5400 / 28000 = 19.29%
  But, should I really be using Purch Price + Repairs instead of just price?

I've calculated Cash on Cash Return as Before Tax Cash Flow / (Purchase costs + Repairs)  3400 / (20000 + 8000) = 12.14%
   Is this the proper calculation?  Should it include the repairs?

Beyond all of that how do the numbers look to you; Deal or no deal?