Hi

With that amount of miles.....Yes....a complete overhaul on the tranny.....Average life span.....subjective.

If they are taken care of well.....Vs not so much.......340K is a really good run......

Get it overhauled before it eats itself up....The 4L80 does not like junk running through the valve body (Aluminum)

Definitely replace the rear main seal while the tranny is out .......If it is leaking a bigger yessssss.

Get all the goodies fixed while you are in there........

As far as cost....Depends on many things.....If the tranny can be saved with seals, clutches and such.....Definitely a fresh converter......Usually bushings and thrust washers.

Shift solenoids for sure.

Get referrals on a good local shop that has a good track record and knows what they are doing.....Stay away from the big box shops.......