Here is a Thread I Posted some time ago.
It outlines the things to look for and the things that you need to do to make an SL match your expectations.
The rule of thumb for expense following ANY 560SL purchase is between $3 to 5000 dollars. Thats if you don't do any of the work yourself.
In regards to your idle problem check the vacuum hoses first.
These cars are now at least 15 years old. A lot of the replaceables have been done piecemeal to sell them. You need to replace everything in a category to prevent revisiting the same problems again and again. Another words not just one hose but all of them. Not just one filter but all of them.
If you catch my drift, you will slowly repair the items that degenerate with age and accomplish a smooth running 560.
People with big Money(Original Cost of 560 was $65K plus) don't, as a rule spend money on maintenance. Gear Heads like you and I do and know the value because we want our stuff to last cause we can't afford new!
New 560sl owner questions