At that mileage, and given your description of the behavior, I'd wonder about one or both ball joints and/or the steering box. Locally, one ball joint on my 560sl was quoted about $225 (parts & labor), and the steering box about $1200 or so. The ball joints are no-maintenance but not everlasting.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)