I'll try with these at least...
Are there any known or common problems with the 3.4 ltr motor? The early motors had occasional headgasket problems resulting in EXTERNAL ONLY coolant leaks. Most should have been fixed by Toyota. Mine was, about 6 months into its life. I got the motor 96K miles later.

Is there anyting to keep my eye on (motor related) when I start looking for my truck? The usual, oil condition, coolant condition. Timing belt/idler-tensioner pulley/water pump replacement if needed. It really helps to do a coolant system pressure check and mechanic's inspection as a matter of course with any used vehicle. Some will do metallurgic/chemical analysis of the oil too.

Any common problems with the R150F trans or V6 Tcase? Nope. Lubrication maybe, std. maintenance.

Any common problems with the front diff? Nope. Same as above.

The rack can wear and develop excessive vertical play, so can the inner tie rod ends. Ball joints. That sort of thing. Should be covered by the mech inspection.