I'd have to agree with Koshinn on this one. Your post reminds me of the joke... how much Healthy Choice ice cream can you eat before it's no longer a healthy choice? How many spare parts can you stock before they shouldn't be spares and its cost effective to just build the rifle?

As of now I haven't had a need for spare parts. Luckily nothing has broken. I have done research on the subject and am still building a few rifles. I think I would be along the lines of:

2-3 sets of gas rings
extractor set (extractor, spring, insert)
ejector set (ejector, spring, 2 retaining pins)
Firing pin, retaining pin
Bolt