I would zone it with 2 systems with the handlers in the Attic and Basement  Still looking around $10-12k with condensers, coils, and sheet metal 

Just hire a tub refinisher they are usually $250-300Also I don't see how you can charge him for the dirty coil.

I nkow I could add an A-coil and condenser to the furnace for central air, but in this price range it doesn't really matter if they use wall ACs.I think I answer my question already, but what do you think?  

BUY quality SINGLE FAMILY homes SFH.that appreciate.4.Always do a home inspection using a certified home inspector ( with utilities turned on) and separately  hire an A.C. company to thoroughly inspect the state of the HVAC ( it might be blowing cold air today , which passes a home inspectors test) but the coils might be shot and the system riddled with leaks and crushed air handling tubes.

Central air is when they put a coil and condenser on the the forced air system so that the duct work is shared between the two. 

That means that in 10% of 50% of cases, we run into a situation where we didn't budget for a compressor replacement that we needed.  10% of 50% is 5% of our projects, and a compressor replacement is between $1000-1800 (depending on whether I need to replace the coil). 

Ducting is already in place, so in that scenario we would normally opt for a centralized heat pump (both heat and air conditioning), but have used gas furnaces with a an air-condtioning exchanger coil in the plenum and the compressor outside.

I had them install the A-coil on the furnace and plumb the lines for future central air in the event I decide to go that direction. 

@Gary Wallace for less than $25 you can buy an electric griddle and you can buy electric table top coils for $12 a piece, which will let you use standard pots.

The special rod he has will be inserted into the sprocket hole and he will crank each side until the coil or spring inside is tensioned correctly for the door to operate smoothly.