Bought a '99 4Runner with 325k, what maintenance to do first?

yea, but he got it go 1 mile....for cheap.....with no idea if it's ever been changed to now.

If it doesn't smell burnt, drive it awhile and feel things out before dumping a bunch of money in it.
(the fluid is not cheap to run a couple gals. through it) If your talking about the screen...if it were clogged it wouldn't be driving already. They're either clogged (trans. is shot) or good to go for quite a bit more.
It looks dam good though.

 

It's amazingly clean!

 

i would get an after market trans cooler and mount and route fluid thru that, abandoning radiator cooler. that is one of the most common failure points and is reasonably cheap and easy to correct ( search strawberry milkshake). Radiators for those are cheap too. $100 at Rockauto for a DEnso radiator (i have 2 on shelf waiting for use, my junk is 5spd so its a non issue for me, just missed the opportunity to swap it out when i had heads off, too lasy/cheap to do it anyway...... )

 

Yeahhhhh, that truck looks like it may have really been takin' care of......you may not need anything right now if fluids look clear.

No leaks? That's commonly what does in older vehicles. Somebody runs 'em out of sumpthin' and sumpthin' seizes up.