There is no telling when the new (used) tranny will go.. so it is hard to say. It all has to do with how it was driven and maintained.
If you end up doing it yourself, it is a risk well taken. Even if the new tranny goes at 120k like your current one, you will have saved a lot on labor costs anyways. It could go to 160k.. or 100k.. you never really know.
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2007 Ford Taurus SE 3.0L OHV V6 (Sold)
1999 Ford Taurus SE 3.0L OHV V6 (Sold)