If the compressor (with clutch and pulley) needs to be replaced, then the accumulator/dryer and line, as well as the oriface and line, also need to be replaced. Then all "O" rings and garter springs need to be replaced. The condenser needs to be pressure tested for leaks and chemically cleaned, as does the evaporator core and manifold lines.
If all of the aforementioned is not done the seller of the compressor will not warrant it.
The total parts and flush kit at retail prices, plus the labor to remove and replace all of the above items, plus the refrigerant and the labor to evacuate the system and recharge it, could easily add up to $1800. This is especially true if R-12 refrigerant is used at about $125 per pound (X 2lbs+).
Can you do it for much less? Yes. Buy the parts and do the labor yourself, or with the assistance of a knowledgable helper. You would have to rent or borrow a vacuum pump and a set of A/C gauges (R-134a gauges, or R-12 gauges - they are different).
If your system WAS charged with R-12 this would be the time to convert to R-134a (it is about $10 per pound). A conversion kit is about $25 and includes new "O" rings, garter springs, and PAG oil or Ester oil for R-134a. A rebuilt compressor is about $250 (your old compressor is a core and will be exchanged), including a new clutch and pulley. The cleaning kit is about $50 through a parts store, or "on-line". The accumulator dryer line and the oriface line are about $250 for both, and are also available "on-line". You will need about 30ozs of R-134a (4 - 8oz cans at about $6 per can).
The $800 estimate to do it yourself is a good estimate. You could get a good used compressor and clutch assembly for a few hundred or less, install it, evacuate the system and recharge it, and hope for the best. Total about $250 to $300. Remember the A/C compressor is SHO specific. It is a NipponDenso 10P15F. Best wishes.
rangerj