Each fluctuates regularly so it's really just about maintaining that balance between the three as best as you can.

I noticed that condos Hoa fees are fairly expensive.

This has worked for me fairly well. 

I hire out property management. 10% of rents is fairly standard for the service.

Many FSBOs will pay commission to a buyer's agent, but that's negotiable.Your Realtor will also be able to give you fairly accurate comps for potential rental income.

How do I know how much to offer them to be fair and still be able to come ahead on my rehab?

His company does good work, they show up on time, and charge a fair rate.

Either the amount of renovation won't increase the after renovation value or sellers are asking fair market price without any room to negotiate.

Also, the house that we rented is a really old farm house, the only source of heat is the wood stove so it's hard to maintain the temperature in the house.

I have a bi-yearly contract with them to maintain the units and they currently have a new policy where they check for mold in the duct work by an intrusive method and they are finding slight mold growth and are recommending this device called Nasa Air Scrubber (~$1200 per unit).