I have a rule of scale.
You can spend the equivalent of the purchase price getting a 20 year up to scratch.
In the W124's case though with proper maintenance the vehicle should easily last to over 500,000 km's.

The big ticket items are the tranny and head gasket.

T.C.O still cheaper than the monthly new car repayments though.

If you can find one with a excellent body and interior it's worth doing all the mechanical stuff.