1995 4Runner 6cyl 4WD, wife is 2nd owner; she got it at 30,000 miles.
Oil changed every 3,000-3,500 miles, other maintenance to schedule.
By oil change, it's used almost a quart (self-refilled at 1/2 qt down).
140,000 miles  -  50/50 city/highway mileage.
Normal hwy trip 2 hrs up Mt Hood, 1.5 hrs back down; reverse trip 2
days later.
Normal city trip is 12 miles in 30 minutes of stop-and-go, reverse 9
hrs later.
Never used for heavy towing, light towing is <500 miles total.
No significant prior problems, no collisions.
BlueBook low-to-avg is $5,150-$5,895.
NADA low-to-average is $5,500-$6,825.

Last week, first temps > 80F of the season, noted a problem.
It overheats after appx. 5 minutes of running at idle in park.
Also overheats after 5 min of <5mph traffic jam on I-205.
Fan runs, radiator properly filled w/ coolant/water mix as prescribed.

What should the mechanics look for first?

What's a reasonable charge for a good (not cheap) shop (e.g., Master
ASE) to do that?

Are there typical failure mode(s)?  Approximate costs?

What's the worst case repair and its approximate cost?

Thank you kindly, all, for your on-topic responses.