There is a "kiddie seat" option for the 107 SL's. Just padded upholstered cushions. I guess you could install seat belts back there as well. Problem is that there is little or no legroom for anyone with legs much longer than 15".

The timing chain does not normally wear out. It does stretch some due to wear of the chain pins. Being over 6 feet long it will develop some stretch over 75K miles. It can be checked with the camshaft valve covers off. There is a procedure where the crank timing marks are lined up, and you then check the camshaft timing marks. The upper guide rails will get dark coffee brown with age and can be observed with the covers off. The four uppers are the ones to change. Most people also have a new chain "rolled" in and the chain tensioner replaced with that job. Unless you confirm that the job was done in the last couple of years, it's good to plan on getting it done. The chain stretches with miles, the guides get brittle with age.