This is what I consider nice to have -
Bolt kit - extractor, ejector plunger and sping, firing pin, bolt roller
Trigger group - springs, trigger pin, trigger, hammer, hammer pin, magazine catch and spring
Gas group - piston, gas valve (plus spring and pin), gas cylinder plug, gas cylinder lock
Receiver group - bolt stop (plus pin and spring), sight elevation knob, windage knob, aperture. roll pin for clip guide
Barrel group - front sight and screw, main spring and recoil guide, flash hider nut, flash hider
Spare butt plate and hand guard with clip
An extra op rod if you have the cash and can find one reasonable
Bolt tool for compressing the ejector to remove the extractor/firing pin
It gets expensive laying in extra parts.
But by all means get the parts for the bolt and trigger group.
You don't really need the bolt tool but it sure make it easier to fix the bolt.
Here are some pics of some of my spare parts.
Just looking at the first picture it would cost me around $500 to buy those parts now.
I think I have a little under $175 in them.
The second picture is just small parts. I'm betting it would cost around $150 to buy them now.
It looks like I do need to get some trigger group parts.
I wish I'd done it three or four years ago.