I just bought a used SCAR 17. It appears in good condition, but the bottom rail screw was stripped out when the previous owner installed the MI rail extension. I plan on replacing it with the KDG SCAR extension so will be ordering a replacement screw.
Are there any other small parts I need to pay attention to/get spares for?
Any other things to look for?
This SCAR has the Striker SEAL SR25 lower, VLTOR stock adapter and Geissele trigger. I noticed it was extremely easy to bumpfire it unintentionally by resetting the trigger while the rifle is still recoiling. I even thought it doubled on me after it happened for the first time.
Is this a function of the lighter Geissele trigger?

This is likely due to mixing the Gtrigger with that lower.  Geissele states not to use the ss trigger with any other lower other than stock.

Scar parts list
Get them here

Thank you.  What is a good aftermarket selector?

I noticed mine was easier to bumpfire with the Geissele trigger after I switched the muzzle device to a flash hider. I kinda wish I could add about a pound of pull to it.

I'm running the Magpul selectors on mine and I'm pretty happy with them, but I've heard some people really like the PMM selectors.

In regards of what to look for, I'd say just inspect for the normal wear stuff you'd normally see on any rifle.

I tried the Magpul ones but didn't like them so they went into the spare parts bin.  I actually like the stock selectors and the stock trigger.  I may put the G trigger back on once I get the NF glass.

First congrats on the SCAR second......

I plan on replacing the MI rail with MREX in FDE.

Before you buy a rail be sure to look at the PMM offerings.