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.
Yes, the Chevrolet K5 Blazer can be equipped with a 396 cubic inch (Ci) engine, but this would typically involve modifications, as the K5 Blazer was not originally offered with that engine option from the factory. The 396 engine was available in various Chevrolet models, primarily in the late 1960s and early 1970s. If you're considering this swap, ensure that the engine mounts, transmission, and other components are compatible for a successful installation.
That would be a matter of opinion. My opinion would be in favor of Chevrolet.
This will not effect the engine in any way
Vacuum leak
That would be the 4L80E transmission.