@Johnny Dang My wife and I just recently replaced our outdoor condensing unit and evaporator coils in the attic of our personal residence here in BR.

Fully stainless and usually the better option in each: electric cooktop instead of coils and 5 burners with heavy grates instead of wire and front load washer dryers.

Ductwork can be cleaned and sanitized no need to replace.. the coil can be cleaned or replaced and that’s all that’s to that the blower motor in your furnace blow air across your cold coil that’s how you get cold air.

Wash your outdoor coil on A/C with a garden hose.These are just minor things you can do that will get you comfortable with your hands, you tube will have videos on each topic, my suggestion would be anticipate what has been missed  and  service it so it does not become a problem later. 

And of course, you would need to make sure the power was off and the coils protected from over spray.

Next hire a pro to clean the heat exchanger and make sure your draft is good in the chimney. the only thing I would recommend Changing system wise is if you have a hot water coil in the boiler.

I also want to add to make sure the coils are cleaned off.

If it's original to the house, or 20+ years old, the cheapest thing to do may be to replace at least the outside unit, inside coil, and lines. 

Once completed we go ahead and remove the water tank, A-coil and condenser unit and store away for safe keeping's.

@Ram V.I do Hvac for a living, usually the bigger the company the more they will charge , take into account equipment brand , the size of the condenser seems about right for the square footage of your home , usually 2 stage furnaces are more expensive I would go with single stage you won’t be saving that much money anyway , ballpark pricing on the equipment would be 1400 for condenser 400 for coil and around 800 for a single stage furnace , that’s cost remember contractors raise the price of the equipment and of course you gotta pay somebody to install it right .