I have never owned an "S" Class vehicle of that vintage, but if you do a search you will find they are a very impressive machine, but unusually complex even for Mercedes-Benz. As a result, they can be costly to maintain. The 600 model has a twelve cylinder engine, which is produced in very limited quantities and therefore parts are going to be even more costly than for other models.
I am curious as to how the automobile has performed for your brother? Has he been satisfied with its reliability and overall cost of ownership? If the car is free, I would not turn it down, but if I was paying the going rate for one, I might first consider what my needs are and what other cars are available for the same cost.
Keep us informed, and if you do buy it or take it as a gift, you can count on getting some help here. But just like those parts, owners of S600 models are kind of rare, and ones with grease under their fingernails from working on their own cars are even more so. Good luck and if this one is not the right car for you, I am sure another Mercedes-Benz will fit fine. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)