Fluid Leaks, Oil Odor (if not able to remove valve cover), timing chain guide rail (clacking) or see if plastic guides if able to remove valve cover, radiator for rust colored debris, steering system (pulling or wheel not on center/returning to center), loose connections in steering system, loose connections of cardon (double) joints of driveshafts, rust, oil burning, exhaust leak test, after 10-15min test drive feel front wheel hubs to see if any are hot to the touch, could be a bad wheel bearing or brakes rubbing, all electronic switches (especially rear window on a 4runner, i think it is common they have issues), etc...
I think all that could be done in about 20-30 minutes. If somebody won't let you check all that, you prob don't wanna buy it. Some people won't want the valve cover removed but if everything else looks good to you and you really want to look under the cover, just tell the person your sold as long as you can take a peek.
Better yet, have a trusted mechanic do a pre-purchase inspection. Maybe $100 cost. Again, if seller not willing to allo that, not sure you would want to buy it.
When I sell a vehilce I'll let somebody check it our or have a mechanic of their choice do it. I know how well I maintain a vehicle, and if their are issues I won'r try to hide them. I'll be upfront and factor that into the asking price.
That's my
(And when buying an older vehilce, I usually always plan on $500-$1000 getting spent within a year. Mostly cause I will always drain/flush/swap all fluids, new filters, belts, hoses, T-Stat (with back flushing water lines).
