Its a 1999 used manual transmission with about 140,000 miles on it already

manual or auto trans?

Depends on the year impreza, model impreza, type of transmission, type of driver. They have 400,000 mile clubs with stock powertrain for subaru's. l had a 2000 Impreza RS Coupe for a little over a year, automatic trans :( , tranny blew up at 46,000 miles. l also had a 2002 WRX, no tranny problems(also put in a built tranny) but they are known for a very weak 2nd gear and have multiple failures.

Subaru has a long history of building automatics that last less than 100K. The manuals however usually last as long if not longer than the engine, which is a lot of miles.

Need more information to give u a full answer, but here it goes.
An automatic should last u as long as anything else o the road, just remember it is a machine & machines do break.
A manual trans is where we need to start clarifying some things:

1) A regular 2.5 liter MT Impreza should last u quite a while, nearly 100K if u do ur proper maintenance.

2) A WRX MT does cause some reasons to be concerned, but if u do ur maintenances, dont have a bunch of modifications & drive it like a normal human being it should last near 100K or more as well.

3) the ST l MT is a pretty strong transmission & providing the same thing about the WRX u should have no issues.

good luck & good driving
thewisetuner