He has been flipping homes for over 5 years and has been successful but in conversation he said he doesn't own any long term rentals but would like to because he uses all the money to buy fix and flips.

Everything that needed fixed from the inspection was fixed by the sellers before we bought. 

esp for fix and flips? 

That said, I can risk another winter with the roof that seems to be holding up decently, only one leak (which I think I can fix) though there is obvious water damage to the sheathing under the shingles, it’s not coming into the rest of the house yet.

I'm currently fixing and flipping a house in Hackettstown, and would like to receive some recommendations.

Don't wait too long because rates are going up, but fix your credit and buy a multi family property. 

What is the issue and how can in fix that?

It is a Fannie Mae ARM with the first year being X and the remaining term of the loan increasing and becoming fixed by 1% the following year for the remainder of the 30 year loan.

Applying for a conventional loan and lender is telling me that it will be 25% down, fixed rate, and 30 year. 

I did not use a property manager but I used a trusted friend to be in charge of the properties; he was able to move into the houses once they closed, take care of the little issues that needed to be fixed up and interview renters (we used Smart Move).