Toyota 4Runner transmission

I am very interested in a 1991 4runner. This vehicle looks well used and
loved. If it wasn’t for the transmission i would say it’s perfect for me (well I
prefer manual but you can’t always win). Okay so anyways he told me this..
“So you know the truck runs and drives but you can’t go to far because the
trans fluid heats up. Will need a trans or At least add and additive. Still there
are trans available for cheap” “It already has a huge trans cooler and lines
Swap the trans takes about 3 hours/ $300” Is this pricing true and can you
diagnose what the problem may be? She’s lifted and has all sorts of added
features. Safari Snorkel, Roof headlights etc. 260 000KM’s $1800. What do
you think? Thanks for the help! .