Your biggest question is what refrigerant to use. R-22 is being phased out and although you can find some condensers, it may not be the best solution. R410 is the new refrigerant and the problem that poses is making sure all the R-22 is out of the refrigeration lines. Check the property description to see the size of the unit that's there. If access to the indoor air handler and outdoor condenser is easy, you may be able to get a new R410 heat pump system for between $5,000 and $7,500 US dollars. You can Google the equipment (condenser and air handler) for a very good idea of the price. The installation is the main variable and that will depend on the ease of installation and how far you want to go with the replacing/cleaning the refrigeration lines.
You might find find that repairing the refrigerant leak in your current system is a far better way to go. If salt air corrosion hasn't destroyed the condenser fins and evaporator coil is clean, your system may be in better condition than you think. At least determine why it's not cooling before jumping into a new system.