well a few things to think about. Yes you can put a coil on top a your furnance and make it work but.............
1. what size is the unit will it be large enough to cool you house.
2. Why is he selling it?
3. Does it work
4. has this unit been installed
5. how old is it
I look at it this way after doing this for over 13yrs. I have to question (if this is your case) putting in used equipment is nothing but problems. You may think your getting a great deal but end up paying more in the long run. If the sytem has been installed somewhere else was it sealed off, was it even working, what kind of condition is the coil in.
If it is a brand new unit never installed then you need to do a load cal. on your house to see how many tons of cooling is needed and to make sure the unit is the right size.
just my 2 cents
if you need help in telling what size it is post braand, model and serial of unit.
Good luck