The SDL is a great car but like any 18+ year old vehicle they do require ongoing maintenance.
The only items I would be prepared for from reading the sellers responses are that you are going to need to replace the Radiator fairly soon as I'm guessing there is a leak at the plastic header/aluminum core seam. Very common. A coolant sealer may plug it for a while but budget for a new rad. While replacing the Rad you probably also want to change the hoses and water pump, Thermostat, and the Sepantine drive belt and Tensioner (also a common maintenace item).
The other item mentioned by the seller is the alternator ground. This shouldn't be an ongoing problem so I would suspect that either the grounding strap between engine & chassis needs changing (no big deal) or the Brush/diode assembly in the alternator may need replacing, assuming the Alternator bearings are still good, otherwise a new alternator may be needed. It is fairly simple to work on the alternator at the same time as overhauling the cooling system as with the bottom hose and Thermostat housing out of the way access is a piece of cake, otherwise
Look for telltale signs of oil in the water or visa-versa that might indicate a bad head gasket or cracked head. Another check is if the car has been run the day before and left overnight, try to squeeze the Top Rad hose before starting the car. If it is soft and pliable OK if hard then you may have a head or head gasket leaking compression into the cooling system.
Do a search here for other common SDL maladies but as always its worth having the car checked out by a specialist before purchase if you can.
Good Luck