I'm thinking this is a $ 300 to $ 400 car due to miles and issue.

Real description of a problem, Trans slips = engine revving beyond what it should for a particular gear.

Some car owners perception, trans slips = low engine power when moving from a stop, car sort of rolls OK when moving. ( Could be misfiring engine from bad plugs , ignition coils, fuel injector ) .

This is the same trans as a 98 + 4 matic E320, have a look in car-part.com.