There is always coolant in the radiator so a leak here will occur with a cold or hot engine.

It could be a fan issue ( clutch fan not engaging , pusher electric fan not turning on )

I'd price out a replacement engine ( like from a S class but be sure to verify , it should be $ as some think SL stuff should command more $ )

I'd put $ 1,500 or less on the car as the market is pretty dead for these otherwise great cars and few are going to take a risk on a car that could be $$$ to fix. Hit them with the cost of a replacement engine as they may have damaged it during prior overheating.

Also, with a 91 you will be getting K jet / CIS injection, this is $$ to repair. A car from the mid 90's will have EFI and will generally be more reliable. Don't discount the 6 cylinder SLs, you can buy a better car for less $ and the 6 is enough for general driving and doesn't run out of breath on the highway.