First, check the EGR valve for leaks and/or proper installation. If that's not it, check for vacuum leaks.
That would be the 4.8L engine.
If the 4.3 Chevrolet engine is new enough to have an EGR valve it would be located in the exhaust pipe where it meets the engine. Some vehicles did not start using this device until after 2000.
ABS brakes should have no effect on engine idle, but a bad power brake vacuum booster would have an effect on engine idle.
That would be the 350 or the 454
Probably more than you would want to spend. You will need the engine,complete wiring harnesses and the engine control module, the computor.
Alot i would assume... I dont know...
Which engine would help.
This will not effect the engine in any way
That would be a matter of opinion. My opinion would be in favor of Chevrolet.
Vacuum leak
That would be the 4L80E transmission.
You would need to call a parts house and they can answer that for you.