Those cars are rock solid. Depends, though, on how much town driving you do (ironically for the name, driving around town is the worst thing).
I'd get the current trans repaired rather than an unknown unit, but it won't make much difference. Should be easy to work on either way.
I had an 03 Crown Vic. Loved it. Ha, you could've had the trans out of it - it'll be at a scrap yard somewhere now. It had... hm, close to 300k km on, no trans work that I know of. So yeah, fix and drive for another 100,000 miles.