I'd hardly call the 129 a money pit. Yes the top hydraulics can leak but are cheap to reseal, labor is the factor here. The top can be raised / lowered manually without much fuss so even a leaking system isn't a problem.

Other systems are somewhat separated, my remote unlock started to fail and not disarm the alarm system. This was solved by unplugging it and using the standard metal key.

In 2011 my 124 K mile 97 SL320 Sport was $ 6,000 and I put about $ 2,500 in parts ( + my labor ) to get it to my standards of reliability. ( I change parts when they get towards end of expected life rather then letting them fail at random. ) Next up is a transmission rebuild because at 132 K miles or so, it is getting doe for a freshen up.

Do you have a "good" car that can be relied on if one of the older ones goes down? Trying to DD an old car that you can't go to the local auto parts and pickup a set of brake or radiator hoses can be stressful.

I'd say the new SL should replace the old SL. The 129 is somewhat heavy and ponderous in town, the 97 C280 I put together for Mom is a much better car round town than my 97 SL320.