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Two year old thread. OP hasn't been on since 2017 so you'll need to shout louder..."Col"

That's right, I have one in mine and it's been as new for years.

I'd just look at changing them all as they're unknown as to their origin and condition. Then they're all new at once.

Order a set of standard factory stuff from Brownells and you'll be fine. You don't need extra power springs.

Why the sear? Is the current one damaged? Don't replace current parts unless you fear failure. The springs are available, the only question is what weight do you want? The heavier ones are for...