It all depends on how the previous owner maintained the vehicle. If they maintained it so it could be ready for long trips at a moment's notice, then it's likely alright. If past maintenance has been spotty, unknown, or just enough to get by, I wouldn't take it out on the highway until all of those things have been sorted out. After things were sorted out I would take it out on the highway by my self on the first few journeys to test it so if some failure happened, I wouldn't have pissed off kids and a wife to deal with. If you have to make a family trip, I believe that the best bet is to rent something.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.