The parts you buy and those you replace depend on 2 things:
1. The condition of your existing parts
2. What you expect from the engine.
Some parts should always be replaced during a rebuild (like bearings), but others are discretionary depending on wear and intended usage (like pushrods).
I don't know how a shop can do parts and labor for a good quality rebuild on a Pontiac for $2000. You'd be hard pressed to do a small block Chevy for that. Here is a basic parts list for what you'll need:
Complete gasket set
Oil pump pickup
Timing chain & gears
Optional parts would include pushrods, rocker arms, and anything you want to upgrade.
And then you'll need the machine work done:
Bore & Hone block
Install freeze plugs & cam bearings
Press piston on rods
Balance rotating assembly
During assembly, you'll then want to rebuild the carb and the distributor to assure a correct level of performance and install all new tuning parts (plugs, wires, cap, rotor, etc).
Most of the engines I build come in at the $3500 range, and I work cheap... better check and see what they're including as part of the "rebuild"...