For the first question here is my thought process. How sure are you on the 200k ARV? How sure are you on the 15k of repairs?
Ken McElroy's book: the ABCs of Real Estate states the asking price should mean nothing to you. He may be asking for 150, but you can do your own homework based on numbers. What is it rented at (if vacant it's worth less, and properties should be assessed based on how they perform). Seller financing can be a good benefit for both parties, you may offer a lower price but give him a higher interest rate and put in writing a plan to refinance with a bank in 2-4 years to pay him in full.
1400 / mo in rent for 150k is less than 1%. Combining that with repairs might not be a winning combo. Do you believe the repair costs and the ARV could also warrant raised rents making the deal worth while?
As for the second question I'm not sure you could get a HELOC unless you refinanced with a bank and had a larger amount of equity in the home. That can be better answered by a lender.
Good luck to you! Don't try and force the numbers to work and don't be afraid to walk away from a bad deal!